iwlwifi: move check health code into iwl-rx.c

Remove check_plcp_health and check_ack_health ops methods, they are
unneeded after iwlegacy driver split. Merge check health code into to
iwl-rx.c and make functions static.

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>