iwlagn: clean up ucode loading

All agn devices behave the same, so there's no
need to go through function pointers for any
of the ucode loading functionality.

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>