mt76x2: fix is_mt7612 routine

Fix is_mt7612 routine since asic version is set in mt76_dev revision
field and not in mt76x2_dev one.
Moreover remove mt76x2_dev rev field since it is never used in the
driver

Fixes: 7bc04215 ('mt76: add driver code for MT76x2e')
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Acked-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>