ramips: mt7621: switch kernel version to 5.4

After all dts and config changes, 4.14 no longer works on mt7621.
Switch it to 5.4 directly.

Signed-off-by: DENG Qingfang <[email protected]>

target/linux/ramips/mt7621/target.mk

@@ -7,6 +7,8 @@ BOARDNAME:=MT7621 based boards
 FEATURES+=nand ramdisk rtc usb minor
 CPU_TYPE:=24kc
 
+KERNEL_PATCHVER:=5.4
+
 define Target/Description
 	Build firmware images for Ralink MT7621 based boards.
 endef