gpio: mvebu: Fix cause computation in irq handler

When switching to regmap, the way to compute the irq cause was
reorganized. However while doing it, a typo was introduced: a 'xor'
replaced a 'and'.

This lead to wrong behavior in the interrupt handler ans one of the
symptom was wrong irq handler called on the Armada 388 GP:
"->handle_irq():  c016303c,
handle_bad_irq+0x0/0x278
->irq_data.chip(): c0b0ec0c,
0xc0b0ec0c
->action():   (null)
   IRQ_NOPROBE set
 IRQ_NOREQUEST set
unexpected IRQ trap at vector 00
irq 0, desc: ee804800, depth: 1, count: 0, unhandled: 0"

Fixes: 2233bf7a ("gpio: mvebu: switch to regmap for register access")
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>