wifi: mt76: optimize ROC for same-channel case

mt76_remain_on_channel() always creates an HT20 chandef and goes
offchannel, even when the ROC channel matches the operating channel.
This unnecessarily narrows bandwidth and triggers beacon stop/restart.

When the ROC channel matches the current operating channel, preserve
the full chandef and skip the offchannel transition, matching the
optimization already present in the scan code.

Extract the shared same-channel detection into mt76_offchannel_chandef()
and use it in both ROC and scan paths.

Link: https://patch.msgid.link/20260309060730.87840-6-nbd@nbd.name
Signed-off-by: Felix Fietkau <nbd@nbd.name>

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