wifi: mt76: mt7996: switch to single multi-radio wiphy

wifi: mt76: mt7996: switch to single multi-radio wiphy

Use generic mt76 chanctx functions and look up phy from vif link.
The driver now uses only a single phy to handle multiple interfaces on
different channels. This is preparation for full MLO support.

Link: https://patch.msgid.link/20250102163508.52945-23-nbd@nbd.name
Signed-off-by: Felix Fietkau <nbd@nbd.name>