The XO-1.5 has a microphone LED designed to indicate to the user when
something is being recorded.

This light is controlled by the microphone bias voltage and it is
currently coming on all the time.

This patch defers the microphone port configuration until when recording
is actually taking place, fixing the behaviour of the LED.
Signed-off-by: Daniel Drake <dsd@laptop.org>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>