ALSA: usb-audio: Convert to the common vmalloc memalloc

The recent change (*) in the ALSA memalloc core allows us to drop the
special vmalloc-specific allocation and page handling.  This patch
coverts to the common code.
(*) 1fe7f397: ALSA: memalloc: Add vmalloc buffer allocation
                  support
    7e8edae3: ALSA: pcm: Handle special page mapping in the
                  default mmap handler

Also, since the SG-buffer-specific PCM ops becomes identical with the
normal PCM ops, unify them again to the single ops, too.

Link: https://lore.kernel.org/r/20191105151856.10785-9-tiwai@suse.de


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