pinctrl: intel: Make use of IRQ_RETVAL()

Instead of using bitwise operations against returned values,
which is a bit fragile, convert IRQ handler to count amount of
GPIO groups, where at least one interrupt happened, and convert
it to returned value by IRQ_RETVAL() macro.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>