mfd: Add missing hunk to change palmas irq to clear on read

During conversion to regmap_irq this hunk was missing being moved
to MFD driver to put the chip into clear on read mode. Also as slave
is now set use it to determine which slave for the register call.

Signed-off-by: Graeme Gregory <gg@slimlogic.co.uk>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>