mac80211: remove useless NULL checks

sdata can't be NULL, and key being NULL is really not possible
unless the code is modified.

The sdata check made a static analyze (klocwork) unhappy because
we would get pointer to local (sdata->local) and only then check
if sdata is non-NULL.

Signed-off-by: Eytan Lifshitz <eytan.lifshitz@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
[remove !key check as well]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>