firmware: arm_scmi: Fix struct kernel-doc warnings in optee transport

Fix the kernel-doc notation for the nested union in struct
scmi_optee_channel to eliminate kernel-doc warnings:

  |  optee.c:130: warning: Excess struct member 'shmem' description
  |			   in 'scmi_optee_channel'
  |  optee.c:131: warning: Function parameter or struct member 'req'
  |			   not described in 'scmi_optee_channel'

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Sudeep Holla <sudeep.holla@arm.com>
Cc: Cristian Marussi <cristian.marussi@arm.com>
Cc:  <linux-arm-kernel@lists.infradead.org>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20240221062157.8694-1-rdunlap@infradead.org


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