[netdrvr] usb/hso: throttle to prevent loss of serial data

Patch to stop loss of characters on the hso modems,
this patch throttles & unthrottles the modem by
not putting out urbs until the tty/line discipline layer
has enough space for newly received packets.
serial ports. This is required for firmware diagnostics
being done at Option.

Signed-off-by: Denis Joseph Barrow <D.Barow@option.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>