wifi: mt76: check chanctx before restoring channel after ROC

wifi: mt76: check chanctx before restoring channel after ROC

mt76_remove_chanctx() sets phy->chanctx to NULL but does not clear
phy->main_chandef. If ROC is later performed on that phy, completion
tries to restore the stale main_chandef channel, programming the
hardware to sit on a channel with no active context.

Add a chanctx check to avoid restoring a channel when no context is
active.

Link: https://patch.msgid.link/20260309060730.87840-4-nbd@nbd.name
Signed-off-by: Felix Fietkau <nbd@nbd.name>