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.comSigned-off-by: NCristian Marussi <cristian.marussi@arm.com>
Signed-off-by: NSudeep Holla <sudeep.holla@arm.com>

drivers/firmware/arm_scmi/optee.c

@@ -106,6 +106,7 @@ enum scmi_optee_pta_cmd {
  * @channel_id: OP-TEE channel ID used for this transport
  * @tee_session: TEE session identifier
  * @caps: OP-TEE SCMI channel capabilities
+ * @rx_len: Response size
  * @mu: Mutex protection on channel access
  * @cinfo: SCMI channel information
  * @shmem: Virtual base address of the shared memory