Fix ALSA available buffersize detection:

Fix ALSA available buffersize detection:

buffer-size = periods * period-size

Previously, buffersize was used for period-size.

This fixes an issue with a dedicated .asoundrc configuring a specific
period-size or buffer-size that has to be exact.
Ardour's device configuration failed in this case.

This has not been an issues since most hardware devices offer a wide
range: 8 < period-size < 262144 ; 16 < buffer-size < 524288.
Only a subset of which (32 .8192) is allowed by Ardour.