iwlagn: verify specific ucode

When we loaded a ucode, there's no point in
checking any one that is present, we know
which one is supposed to be present so also
verify that it is exactly the right one.

That also simplifies the code and makes it
faster since it doesn't have to check all.

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>