gpio: dwapb: Fix error handling in dwapb_gpio_probe()

If dwapb_gpio_add_port() fails in dwapb_gpio_probe(),
gpio->clk is left undisabled.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>