iwlagn: consolidate the API that sends host commands and move to transport

Now, there are only two functions to send a host command:
* send_cmd that receives a iwl_host_cmd
* send_cmd_pdu that builds the iwl_host_cmd itself and received flags

The flags CMD_ASYNC / CMD_SYNC / CMD_WANT_SKB are not changed by the API
functions.

Kill the unused flags CMD_SIZE_NORMAL / CMD_NO_SKB on the way.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>