USB: serial: omninet: fix reference leaks at open

commit 30572418b445d85fcfe6c8fe84c947d2606767d8 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>
Signed-off-by: Willy Tarreau <w@1wt.eu>