wifi: mt76: add offchannel check to mt76_roc_complete

wifi: mt76: add offchannel check to mt76_roc_complete

mt76_roc_complete() unconditionally calls __mt76_set_channel() to
restore the operating channel. The scan equivalent mt76_scan_complete()
checks phy->offchannel first, skipping the restore if the phy is
already back on-channel.

Without this check, ROC completion performs a redundant full hardware
channel switch when something has already moved the phy back.

Link: https://patch.msgid.link/20260309060730.87840-3-nbd@nbd.name
Signed-off-by: Felix Fietkau <nbd@nbd.name>