USB: serial: fix potential use-after-free after failed probe

Fix return value in probe error path, which could end up returning
success (0) on errors. This could in turn lead to use-after-free or
double free (e.g. in port_remove) when the port device is removed.

Fixes: c706ebdf ("USB: usb-serial: call port_probe and port_remove
at the right times")
Cc: stable <stable@vger.kernel.org>	# v2.6.31
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Greg Kroah-Hartman <greg@kroah.com>