power: smb135x-charger: Store the initial IRQ status register value

There is a possibility that an IRQ status change may not registered
if the IRQ status value (prev_val) is not updated while reading the
initial state. Make sure that the IRQ status is updated in the irq
handler structure.

Change-Id: I2acbe4e9cfc12abc64f81d65cfc0863ea8004adb
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>