gpio/langwell_gpio: ack the correct bit for langwell gpio interrupts

The wrong bit was masked when acking langwell gpio interrupts.

Reason for maskig the wrong bit was probably because__ffs() and ffs() functions
return bit indexes differently (0..31 vs 1..32)

This fixes langwell based devices from hanging when a gpio interrupt is
triggered and undoes the breakage which occurred in change set
732063b92bb727b27e61580ce278dddefe31c6ad

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/gpio/langwell_gpio.c

@@ -223,7 +223,7 @@ static void lnw_irq_handler(unsigned irq, struct irq_desc *desc)
 		gedr = gpio_reg(&lnw->chip, base, GEDR);
 		pending = readl(gedr);
 		while (pending) {
-			gpio = __ffs(pending) - 1;
+			gpio = __ffs(pending);
 			mask = BIT(gpio);
 			pending &= ~mask;
 			/* Clear before handling so we can't lose an edge */