USB: EHCI: handle isochronous underruns with tasklets

This patch updates the iso_stream_schedule() routine in ehci-sched.c
to handle cases where an underrun causes an isochronous endpoint's
queue to empty out, but the client driver wants to maintain
synchronization with the device (i.e., the URB_ISO_ASAP flag is not
set).  This could not happen until recently, when ehci-hcd switched
over to completing URBs in a tasklet.

(This may seem like an unlikely case to worry about, but underruns are
all too common with the snd-usb-audio driver, which doesn't use
URB_ISO_ASAP.)

As part of the fix, some URBs may need to be given back when they are
submitted.  This is necessary when the URB's scheduled slots all fall
before the current value of ehci->last_iso_frame, and as an
optimization we do it also when the slots all fall before the current
frame number.

As a second part of the fix, we may need to skip some but not all of
an URB's packets.  This is necessary when some of the URB's scheduled
slots fall before the current value of ehci->last_iso_frame and some
of them fall after the current frame number.  A new field
(first_packet) is added to struct ehci_iso_sched, to indicate how many
packets should be skipped.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
CC: Ming Lei <tom.leiming@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>