linux-pulseaudio: Fix the default req. buffer len

This changes the the requested buffer length to 25ms.
linux-pulseaudio: Use defaults for pa_buffer_attr

Although these values aren't used in capture mode, set these to the
default values just to be safe.
linux-pulseaudio: Force stereo if unknown layout

This adds a check to change the capture settings to use 2 channels when
a channel number is encountered that would otherwise be interpreted as
SPEAKERS_UNKNOWN.