gpio: mlxbf3: add an error code check in mlxbf3_gpio_probe

Clang static checker warning: Value stored to 'ret' is never read.
bgpio_init() returns error code if failed, it's better to add this
check.

Fixes: cd33f216 ("gpio: mlxbf3: Add gpio driver support")
Signed-off-by: Su Hui <suhui@nfschina.com>
[Bartosz: add the Fixes: tag]
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>