firmware: arm_scmi: Fix response size warning for OPTEE transport

firmware: arm_scmi: Fix response size warning for OPTEE transport

Some protocols check the response size with the expected value but optee
shared memory doesn't return such size whereas it is available in the
optee output buffer.

As an example, the base protocol compares the response size with the
expected result when requesting the list of protocol which triggers a
warning with optee shared memory:

arm-scmi firmware:scmi0: Malformed reply - real_sz:116  calc_sz:4  (loop_num_ret:4)

Save the output buffer length and use it when fetching the answer.

Link: https://lore.kernel.org/r/20220624074549.3298-1-vincent.guittot@linaro.org
Reviewed-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>