mfd: wcd9xxx: clear unhandled interrupt request properly

Current irq handler writes INTR_STATUS registers to clear irqs incorrectly
when unhandled irq is detected while it has to write INTR_CLEAR instead.
Also INTR_MODE register needs to be updated after clearing interrupt
requests if hardware is running off i2c.

Fix irq handler to write correct reset value to INTR_CLEAR registers along
with INTR_MODE register update for i2c mode.

Change-Id: Icc6837c3937a5c1251dd1821154ed4cb676ec063
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>