power: smb1390: Fix IRQ status for cp_irq property

The cp_irq property is designed to report all fired IRQs cached starting
from last read. However, most of the time, all registered IRQs are
returned instead, due to a SW bug.

Fix it.

Change-Id: Ic6e6c31b8e8c383b3400d0c9876aad6a8315a3ca
Signed-off-by: Harry Yang <harryy@codeaurora.org>