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