ALSA: scarlett2: Add power status control

Add a control to retrieve the power status from the interface
(bus-powered, external-powered, or insufficient power).

Mark the new scarlett2_notify_power_status() function with
__always_unused until it gets used when the Gen 4 notification
callback function arrays are added.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/d9806bc41adc45b1c19749562fec7765ba24351d.1703612638.git.g@b4.vu