ALSA: usb-audio: Drop CONFIG_PM ifdefs

Practically seen, CONFIG_PM is almost mandatory.
Let's drop the ugly ifdef lines and simplify the code.

Link: https://lore.kernel.org/r/20211202084053.18201-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>