firmware: arm_scmi: Refactor xfer in-flight registration routines

Move the whole xfer in-flight registration process out of scmi_xfer_get
and while at that, split the sequence number selection steps from the
in-flight registration procedure itself.

No functional change.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20230118121426.492864-2-cristian.marussi@arm.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>