nl80211: fix locking in nl80211_set_tx_bitrate_mask()

This accesses the wdev's chandef etc., so cannot safely
be used without holding the lock.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Link: https://lore.kernel.org/r/20220506102136.06b7205419e6.I2a87c05fbd8bc5e565e84d190d4cfd2e92695a90@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>