Use a more correct calculation for highspeed bit times.

  http://bugzilla.kernel.org/show_bug.cgi?id=3604

This sort if thing might start to make a difference now that the high
speed periodic scheduler is more complete -- and even getting used.
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>