firmware: arm_scmi: Fix signed error return values handling

Handle signed error return values returned by simple_write_to_buffer().
In case of an error, return the error code.

Fixes: 3c3d818a ("firmware: arm_scmi: Add core raw transmission support")
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Sukrut Bellary <sukrut.bellary@linux.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Tested-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/20230718085529.258899-1-sukrut.bellary@linux.com


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