Make audio FIFO read/write contracts more strict

Signed-off-by: Michael Bradshaw <mjbshaw@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
lavd/v4l2: honor previously selected input channel

An input channel could have been previously set with another application, like
v4l2-ctl, so if no input channel is specified use the previosly selected one.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>