mfd: lp8788-irq: Uninitialized variable in irq handler

Instead to being true/false, the "handled" is true/uninitialized.
Presumably this doesn't cause that many problems in real life because
normally we handle the IRQ.

Fixes: eea6b7cc ('mfd: Add lp8788 mfd driver')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Milo Kim <milo.kim@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>