firmware: arm_scmi: Clear stale xfer->hdr.status

Stale error status reported from a previous message transaction must be
cleared before starting a new transaction to avoid being confusingly
reported in the following SCMI message dump traces.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20221222183823.518856-2-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>