iwlagn: fix non-5000+ build

When building 4965 without 5000+ there were a
lot of build errors due to functions being used
that weren't even compiled in. To fix this move
some code around and only compile the HCMD code
for 5000+ series as it's not used for 4965.

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>