sound: sgio2audio/pdaudiocf/usb-audio: initialize PCM buffer

When allocating the PCM buffer, use vmalloc_user() instead of vmalloc().
Otherwise, it would be possible for applications to play the previous
contents of the kernel memory to the speakers, or to read it directly if
the buffer is exported to userspace.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Cc: <stable@kernel.org>

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