USB: keyspan: Fix oops

If we get a data URB back from the hardware after we have put the tty to
bed we go kaboom. Fortunately all we need to do is process the URB
without trying to ram its contents down the throat of an ex-tty.

Cc: stable <stable@kernel.org>
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>