wifi: iwlwifi: mvm: Add locking to the rate read flow

The rs_drv_get_rate flow reads the lq_sta to return the optimal rate
for tx frames. This read flow is not protected thereby leaving
a small window, a few instructions wide, open to contention by an
asynchronous rate update. Indeed this race condition was hit and the
update occurred in the middle of the read.

Fix this by locking the lq_sta struct during read.

Signed-off-by: Ariel Malamud <ariel.malamud@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230514120631.b52c9ed5c379.I15290b78e0d966c1b68278263776ca9de841d5fe@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>