wifi: cw1200: remove RCU STA pointer handling in TX

We can call this in one of two ways: through mac80211, where
we're already in an RCU read-side critical section, or from
some other code in the driver where this pointer can only be
NULL. In any case, we get a 'free' already protected pointer
to the sta through info->control.sta, so we can use it on
the stack without any further protection.

Remove the rcu_dereference() and critical section.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220904212910.e5bc20dd17bf.Ib570ff7fde33c2b6eddef493a3541fa04eb47181@changeid