`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 <anton@ryzhov.me>