Remove __packed attribute from struct scmi_event_header.

Link: https://lore.kernel.org/r/20200710133919.39792-2-cristian.marussi@arm.comSigned-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>