mt7601u: remove a warning in mt7601u_efuse_physical_size_check()

Fix the following sparse warning in mt7601u_efuse_physical_size_check:
- drivers/net/wireless/mediatek/mt7601u/eeprom.c:77:27: warning:
  Variable length array is used

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Acked-by: Jakub Kicinski <kubakici@wp.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>