pcm:plugin: Fix sound capture via MMAP access

Distorted sound is heard if the capture device of an ioplug plugin is used,
which is accessed through any ALSA plugin (like copy, linear, ...) via MMAP access.
E.g. aplay -> loop -> copy -> arecord -M -> aplay -> hw:0
As mentioned in the ALSA API (see pcm/pcm.c:942):
The function #snd_pcm_avail_update() have to be called
before any mmap begin+commit operation.

Signed-off-by: Timo Wischer <twischer@de.adit-jv.com>
Signed-off-by: Mounesh Sutar <sutar.mounesh@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>