tty: sc16is7xx: remove retval from gpiochip_remove()

We are trying to smoke out the use of the return value from
gpiochip_remove() from the kernel, this has been missed.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>