HID: i2c-hid: create a helper for SET_POWER command

Another case where creating a dedicated helper allows for cleaner code that
shows exactly what communication happens with the device when toggling its
power.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Tested-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>