iwlwifi: move remaining iwl-agn-rx.c code into iwl-rx.c

There is no need to have separate iwl-agn-rx.c file after iwlegacy
split.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>