iwlwifi: fix memory leak if opmode fails to init

If drv->op_mode is NULL after trying to init the
opmode, we go to the wrong label. Fix this, and
clean up the code a bit.

Reviewed-by: Gregory Greenman <gregory.greenman@intel.com>
Reviewed-by: Guy Cohen <guy.cohen@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>