USB: serial: fix tty-device error handling at probe

Add missing error handling when registering the tty device at port
probe. This avoids trying to remove an uninitialised character device
when the port device is removed.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Reported-by: Takashi Iwai <tiwai@suse.de>
Cc: stable <stable@vger.kernel.org>	# v2.6.12
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Greg Kroah-Hartman <greg@kroah.com>