iwlwifi: add wrappers for command sending

Add wrappers to send commands from the DVM
op-mode (which essentially consists of the
current driver). This will allow us to move
specific sanity checks there.

Also, this removes iwl_trans_send_cmd_pdu()
since that can now be taken care of in the
DVM-specific wrapper.

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>