Add SCMI type handling to pack/unpack_scmi_header common helper functions.
Initialize hdr.type properly when initializing a command xfer.

Link: https://lore.kernel.org/r/20210803131024.40280-2-cristian.marussi@arm.comSigned-off-by: NCristian Marussi <cristian.marussi@arm.com>
Signed-off-by: NSudeep Holla <sudeep.holla@arm.com>