gpiolib: of: Make of_gpio_spi_cs_get_count static

Fix sparse warning:

drivers/gpio/gpiolib-of.c:35:5: warning:
 symbol 'of_gpio_spi_cs_get_count' was not declared. Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20191130012828.14504-1-yuehaibing@huawei.com
Signed-off-by: Mark Brown <broonie@kernel.org>