iwlwifi: testing the wrong variable in iwl_add_bssid_station()

The intent here is to test that "sta_id_r" is a valid pointer.  We do
this same test later on in the function.

Btw iwl_add_bssid_station() is called from two places and "sta_id_r" is
a valid pointer from both callers.

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