iwlagn: generically provide iwl_trans_send_cmd_pdu

There's no need to have the transport layer have a
callback for iwl_trans_send_cmd_pdu() since it is
just a generic wrapper around iwl_trans_send_cmd().

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>