firmware: arm_scpi: fix send_message and sensor_get_value for big-endian

scpi_process_cmd converts the status word from little endian to cpu
endianness. However scpi_send_message again does the conversion which is
wrong and shows up as a bug only when running in big-endian kernel.

Similarly scpi_sensor_get_value passes the sensor index in the cpu
endianness to SCP which results in SCPI_ERR_RANGE in big-endian mode.

This patch fixes the above mentioned issue for big-endian kernel.

Acked-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>