gpio: brcmstb: Make .remove() obviously always return 0

priv cannot be NULL because brcmstb_gpio_probe() calls
platform_set_drvdata() with a non-NULL argument, so the check for !priv can
be dropped.

Also remove the variable ret that is only used to hide a bit that in the
end zero is returned.

This is a preparation for making platform remove callbacks return void.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>