mfd: Still check other interrupts if we get a wm831x touchscreen IRQ

It is possible that we will see another interrupt triggering at the same
time as the touchscreen interrupts so it's still worth checking other
possible sources. Almost all of the win from the fast path comes from only
needing to read the primary register and saving the I/O costs.

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