iwlagn: remove the callback in host commands

Before this patch, the upper layer could register a callback for each
host command. This mechanism allowed the upper layer to have
different callbacks for the same command ID. In fact, it wasn't used
and the rx_handlers is enough: same callback for all the command with
a specific command ID.

The iwl_send_add_station needs the access the command that was sent
while handling the response (regardless if the command was sent in
SYNC or ASYNC mode). So now, all the handlers receive the host
command that was sent. This implies a change in the handler signature.

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>