USB: serial: console: fix use-after-free after failed setup

Make sure to reset the USB-console port pointer when console setup fails
in order to avoid having the struct usb_serial be prematurely freed by
the console code when the device is later disconnected.

Fixes: 73e487fdb75f ("[PATCH] USB console: fix disconnection issues")
Cc: stable <stable@vger.kernel.org>	# 2.6.18
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>
