ramips: fix kernel oops in `mt7621_nfc_write_page_hwecc`

`mt7621_nfc_write_page_hwecc` may be called with `buf=NULL`, but
`mt7621_nfc_check_empty_page` always tries to read it.
That caused Oops:
`Unable to handle kernel paging request at virtual address 00000000`

Fixes: FS#3416
Signed-off-by: Anton Ryzhov <[email protected]>

target/linux/ramips/patches-5.4/0300-mtd-rawnand-add-driver-support-for-MT7621-nand-flash.patch

@@ -47,7 +47,7 @@ Signed-off-by: Weijie Gao <[email protected]>
  obj-$(CONFIG_MTD_NAND_TEGRA)		+= tegra_nand.o
 --- /dev/null
 +++ b/drivers/mtd/nand/raw/mt7621_nand.c
-@@ -0,0 +1,1348 @@
+@@ -0,0 +1,1350 @@
 +// SPDX-License-Identifier: GPL-2.0
 +/*
 + * MediaTek MT7621 NAND Flash Controller driver
@@ -1155,9 +1155,11 @@ Signed-off-by: Weijie Gao <[email protected]>
 +	uint32_t i, j;
 +	u8 *oobptr;
 +
-+	for (i = 0; i < mtd->writesize; i++)
-+		if (buf[i] != 0xff)
-+			return 0;
++	if (buf) {
++		for (i = 0; i < mtd->writesize; i++)
++			if (buf[i] != 0xff)
++				return 0;
++	}
 +
 +	for (i = 0; i < nand->ecc.steps; i++) {
 +		oobptr = oob_fdm_ptr(nand, i);