Staging: quatech_usb2: implement open functionality

Part two of the driver implementation has finally got done. It
implements the rest of open and the callback for reads from the box. I
seem to have finally found a structure that can be made to work, with a
single set of URBs for bulk read and write for the whole device, which
are used by all the ports. I've rationalised a few things, but there
will still be a lot of clean-up needed.

This one definitely can panic the kernel when a port is opened for
reading, but I'm off on holiday so I thought I'd post where I have got
to so far. I haven't tried to debug why it panics with a null pointer
dereference yet.
Signed-off-by: Richard Ash <richard@audacityteam.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>