commit 30572418 upstream.

This driver needlessly took another reference to the tty on open, a
reference which was then never released on close. This lead to not just
a leak of the tty, but also a driver reference leak that prevented the
driver from being unloaded after a port had once been opened.

Fixes: 4a90f09b ("tty: usb-serial krefs")
Signed-off-by: Johan Hovold <johan@kernel.org>
[bwh: Backported to 3.2:
 - The 'serial' variable is still needed for other initialisation
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>