iwlwifi: reduce sta_lock hold time in TX

When we TX, we hold the sta_lock for a long
time, a lot of which isn't needed. Reduce
the time we hold the lock. Note that this
doesn't really change anything as the code
is already under the other spinlock, but it
makes the code easier to read.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>