gpio: pca953x: suppress interrupts warning when not applicable

Don't warn about missing interrupts support when the parent interrupt is
not defined. Enabling interrupts support would not make it work anyway.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>