iio: adc: ti-ads7950: Fix build error without CONFIG_GPIOLIB

When building with CONFIG_GPIOLIB is not set
gcc warns this:

drivers/iio/adc/ti-ads7950.c:75:19: error: field chip has incomplete type
  struct gpio_chip chip;
                   ^~~~
drivers/iio/adc/ti-ads7950.c: In function ti_ads7950_set:
drivers/iio/adc/ti-ads7950.c:409:32: error: implicit declaration of function gpiochip_get_data; did you mean acpi_get_data? [-Werror=implicit-function-declaration]
  struct ti_ads7950_state *st = gpiochip_get_data(chip);
                                ^~~~~~~~~~~~~~~~~
                                acpi_get_data

Reported-by: Hulk Robot <hulkci@huawei.com>
Fixes: c97dce79 ("iio: adc: ti-ads7950: add GPIO support")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Reviewed-by: Justin Chen <justinpopo6@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>