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

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

commit ca4383a3947a83286bc9b9c598a1f55e867871d7 upstream.

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: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Reported-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Johan Hovold <johan@kernel.org>
Acked-by: Greg Kroah-Hartman <greg@kroah.com>
[bwh: Backported to 3.2:
 - Adjust context
 - No need to clean up autopm]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>