USB: EHCI: add missing frame -> microframe conversion

This patch (as1407) fixes a bug in ehci-hcd's isochronous scheduler.
All its calculations should be done in terms of microframes, but for
full-speed devices, sched->span is stored in frames.  It needs to be
converted.

This fix is liable to expose problems in other drivers.  The old code
would accept URBs that should not have been accepted, so drivers have
had no reason to avoid submitting URBs that exceeded the maximum
schedule length.  In an attempt to partially compensate for this, the
patch also adjusts the schedule length from a minimum of 256 frames up
to a minimum of 512 frames.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: David Brownell <david-b@pacbell.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>