firmware: arm_scpi: fix endianness of dev_id in struct dev_pstate_set

scpi_device_{g,s}et_power_state correctly handles the conversion of
endianness for dev_id using cpu_to_le16. However dev_id is declared
as u16 in struct dev_pstate_set which is incorrect.

This patch fixes the endianness of dev_id in dev_pstate_set structure.

Fixes: 37a441dc ("firmware: arm_scpi: add support for device power state management")
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>