ALSA: usb-audio: Factor out DSD bitrev copy function

Just minor code refactoring.  Like DOP DSD code, it can be better in a
separate function for code readability.

Link: https://lore.kernel.org/r/20210601162457.4877-5-tiwai@suse.de


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