USB: serial: omninet: fix reference leaks at open

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")
Cc: stable <stable@vger.kernel.org>	# 2.6.28
Signed-off-by: Johan Hovold <johan@kernel.org>