iwlwifi: simplify code in iwlagn_key_sta_id

Simplify the code by returning directly in the
error case and replacing the switch/if with a
single if statement.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>