wifi: mt76: mt7915: fix omac index assignment after hardware reset

Reset per-phy mac address slot mask in order to avoid leaking entries.

Fixes: 8a55712d ("wifi: mt76: mt7915: enable full system reset support")
Link: https://patch.msgid.link/20241230194202.95065-12-nbd@nbd.name


Signed-off-by: Felix Fietkau <nbd@nbd.name>