USB: uss720 fixup refcount position

My testprog do a lot of bitbang - after hours i got following warning and my machine lockups:
WARNING: at /build/buildd/linux-2.6.38/lib/kref.c:34
After debugging uss720 driver i discovered that the completion callback was called before
usb_submit_urb returns. The callback frees the request structure that is krefed on return by
usb_submit_urb.

Signed-off-by: Peter Holik <peter@holik.at>
Acked-by: Thomas Sailer <t.sailer@alumni.ethz.ch>
Cc: stable <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>