USB generic driver
When capturing audio data, we do not know beforehand how many samples
the device sends per frame, so we have to use URBs that are as short as
possible to make sure that we can handle period boundaries without any
additional latencies.

Furthermore, the total count of URBs submitted doesn't matter when
capturing, so we can just use the maximum number.
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>