iwlwifi: assume vif is NULL for internal scans and non-NULL otherwise

The current practice of checking vif for NULL in one place but not
another seems to confuse some static checkers, smatch in particular.
Since vif will only be NULL in the case of internal scans, adjust the
checks accordingly.

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