pinctrl: sp7021: fix unused function warning

pinctrl: sp7021: fix unused function warning

sppctl_gpio_inv_get is only used from the debugfs code inside
of an #ifdef, so we get a warning without that:

drivers/pinctrl/sunplus/sppctl.c:393:12: error: 'sppctl_gpio_inv_get' defined but not used [-Werror=unused-function]
  393 | static int sppctl_gpio_inv_get(struct gpio_chip *chip, unsigned int offset)
      |            ^~~~~~~~~~~~~~~~~~~

Replace the #ifdef with an IS_ENABLED() check that avoids the warning.

Fixes: aa74c44be19c ("pinctrl: Add driver for Sunplus SP7021")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20221215163822.542622-1-arnd@kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>