mt76x0: correct type for eeprom gain value

Change type to u8 to allow sanity check agaist 0xff;

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: e87b5039 ("mt76x0: eeprom files")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>