mfd: cpcap: Use ack_invert interrupts

We should use ack_invert as the int_read_and_clear() in the Motorola
kernel tree does "ireg_val & ~mreg_val" before writing to the mask
register.

Fixes: 56e1d40d ("mfd: cpcap: Add minimal support")
Tested-by: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>