generic: backport missing helper for phy interfaces

The function was defined before in the patch
"hack-5.15/795-backport-phylink_pcs-helpers.patch". However, the hack
did not move to 6.1. Instead of using the hack do the backport of
upstream accepted version.

Fixes errors in the form of:
  make[8]: Entering directory '/home/nick/openwrt/build_dir/target-mipsel_24kc_musl/linux-ramips_mt7621/linux-6.1.29'
    CC      drivers/net/pcs/pcs-mtk-lynxi.o
  drivers/net/pcs/pcs-mtk-lynxi.c: In function 'mtk_pcs_lynxi_config':
  drivers/net/pcs/pcs-mtk-lynxi.c:160:30: error: implicit declaration of function 'phylink_get_link_timer_ns' [-Werror=implicit-function-declaration]
    160 |                 link_timer = phylink_get_link_timer_ns(interface);
        |                              ^~~~~~~~~~~~~~~~~~~~~~~~~
  cc1: all warnings being treated as errors

Fixes: fa79baf4a6e2 ("generic: copy backport, hack, pending patch and config from 5.15 to 6.1")
Signed-off-by: Nick Hainke <[email protected]>

target/linux/generic/backport-6.1/700-net-phylink-add-phylink_get_link_timer_ns-helper.patch

@@ -0,0 +1,48 @@
+From 9c5a170677c3c8facc83e931a57f4c99c0511ae0 Mon Sep 17 00:00:00 2001
+From: "Russell King (Oracle)" <[email protected]>
+Date: Thu, 27 Oct 2022 14:10:37 +0100
+Subject: [PATCH] net: phylink: add phylink_get_link_timer_ns() helper
+
+Add a helper to convert the PHY interface mode to the required link
+timer setting as stated by the appropriate standard. Inappropriate
+interface modes return an error.
+
+Signed-off-by: Russell King (Oracle) <[email protected]>
+Signed-off-by: Jakub Kicinski <[email protected]>
+---
+ include/linux/phylink.h | 24 ++++++++++++++++++++++++
+ 1 file changed, 24 insertions(+)
+
+--- a/include/linux/phylink.h
++++ b/include/linux/phylink.h
+@@ -614,6 +614,30 @@ int phylink_speed_up(struct phylink *pl)
+ 
+ void phylink_set_port_modes(unsigned long *bits);
+ 
++/**
++ * phylink_get_link_timer_ns - return the PCS link timer value
++ * @interface: link &typedef phy_interface_t mode
++ *
++ * Return the PCS link timer setting in nanoseconds for the PHY @interface
++ * mode, or -EINVAL if not appropriate.
++ */
++static inline int phylink_get_link_timer_ns(phy_interface_t interface)
++{
++	switch (interface) {
++	case PHY_INTERFACE_MODE_SGMII:
++	case PHY_INTERFACE_MODE_QSGMII:
++	case PHY_INTERFACE_MODE_USXGMII:
++		return 1600000;
++
++	case PHY_INTERFACE_MODE_1000BASEX:
++	case PHY_INTERFACE_MODE_2500BASEX:
++		return 10000000;
++
++	default:
++		return -EINVAL;
++	}
++}
++
+ void phylink_mii_c22_pcs_decode_state(struct phylink_link_state *state,
+ 				      u16 bmsr, u16 lpa);
+ void phylink_mii_c22_pcs_get_state(struct mdio_device *pcs,