mfd: Fix off by one in WM831x IRQ code

The GPIO IRQs aren't the first IRQs defined, we need to subtract the base
for the GPIOs as well to use them for array indexes.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>