gpio: crystalcove: Use for_each_set_bit() in IRQ handler

This simplifies and standardizes the AB IRQ handler by using
the for_each_set_bit() library function.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>