V4L/DVB (5917): ivtv: improve mailbox responsiveness.

First try polling for the result of a mailbox command, then
switch to a test/sleep loop. Also reduce the sleep time from 10 ms
to 1 ms. Improves the responsiveness of the mailbox handling.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>