ipack: ipoctal: fix tty-registration error handling

ipack: ipoctal: fix tty-registration error handling

Registration of the ipoctal tty devices is unlikely to fail, but if it
ever does, make sure not to deregister a never registered tty device
(and dereference a NULL pointer) when the driver is later unbound.

Fixes: cb4c81cccd76 ("Staging: ipack/devices/ipoctal: Check tty_register_device return value.")
Cc: stable@vger.kernel.org      # 3.7
Acked-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20210917114622.5412-4-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>