ath9k: cleanup: remove unneeded null check

We dereference "wmi" on the line before and also when we initialize "ah".
This check has always been after a dereference since the first commit a
couple months ago.  Looking through the code, it looks like "wmi" can't
actually be null here so I just removed the check.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>