firmware: arm_scmi: Fix missing kernel-doc in optee

Add the missing structure field `rx_len` description.

Link: https://lore.kernel.org/r/20220817172731.1185305-2-cristian.marussi@arm.com


Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>