Intensive tests with USB audio devices connected to a musb host port
have shown reproducible pops and clicks in both the playback and the
capture stream.

These are related to how the early_tx hrtimer is set up, and it turns
out they can be fixed by reducing the timer's slack value from 40 to
25 us. Also, when the callback is ran without taking action, it should
be rescheduled 20 us later instead of 50 us.
Reported-and-tested-by: NSven Neumann <neumann@teufel.de>
Signed-off-by: NDaniel Mack <daniel@zonque.org>
Signed-off-by: NFelipe Balbi <balbi@ti.com>