This fixes call audio on the devkit.  The stock default sample rate is
44.1 kHz and PulseAudio wants a multiple of 8 kHz due to Haegtesse.
Confusion ensues because the hardware restricts the available sample
rate in either the ADC or DAC once the other side is opened.
PulseAudio assumes that the list of available sample rates never
changes.  We work around this by making the default sample rate 48
kHz, a multiple of 8 kHz.