Increase Unicens driver PCM buffer size

The Unicens driver's PCM buffer is smaller than 'regular' ALSA card's buffers (currently 252, instead of at least 1024 for other cards).

As the softmixer use this buffer to wait for audio frames to write, a smaller buffer size will increase the number of softmixer's writes, e.g. increasing its CPU consumption.

It would be great to increase this PCM buffer size to avoid that.