mt76: mt7915: rely on mt76_connac_get_phy utilities

Rely on mt76_connac_get_phy_mode and mt76_connac_get_he_phy_cap utility
routines in mt7915 driver and remove duplicated code.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>