gpio: dwapb: Switch to use fwnode_irq_get()

We have open coded variant of fwnode_irq_get() in dwapb_get_irq().
Replace it with a simple call.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>