plugin: dynamically update avail_min on slave

mmapped capture access on some plugins can fetch data from
slave in the 'background'. A subsequent snd_pcm_wait waits
for too long time to reach avail_min threshold again.
Waiting too long leads to xruns on other devices waiting for
the capture data.
As a fix the avail_min on slave is recalculated dynamically.

V2: updated patch to fix within 80 characters per line

Signed-off-by: Andreas Pape <apape@de.adit-jv.com>
Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>