wifi: mt76: mt7915: add missing chanctx ops

Looks like this was missed in the initial patch that made
the conversion to the emulated chanctx drivers.

Fixes: 0a44dfc0 ("wifi: mac80211: simplify non-chanctx drivers")
Tested-by: James Courtier-Dutton <james.dutton@gmail.com>
Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>