iwlagn: simplify the iwl_device_cmd layout

This simplifies both the transport layer and the upper layer.

Kill the union in the device command, which avoids the funny syntax
we had: cmd->cmd.payload.

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>