USB: EHCI: start new isochronous streams ASAP

This patch changes the initial delay before the startup of a newly
scheduled isochronous stream.  Currently the stream doesn't start
for at least 5 ms (40 microframes).  This value is just an estimate;
it has no real justification.

Instead, we can start the stream as soon as possible after the
scheduling computations are complete.  Essentially this requires
nothing more than reading the frame counter after the stream is
scheduled, instead of before.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>