power: smb135x-charger: fix wakeup source handling in suspend

The system fails to suspend if it sees a wake source during
it's suspend process. The charger driver holds a wake source
for every i2c read/write operation which fails the system suspend.
Add a flag to track the device suspend and avoid holding the wake
source when this flag is set.

While at it, also clean up the wake source handling.

CRs-Fixed: 1042700
Change-Id: Ib1d4348d416a662967ef57a73a8e79a76205ade6
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>