power: qpnp-qg: Fix the check in PON SOC calculation

The current check always returns a positive value due
to an unsigned in wrap-around. Fix it.

Change-Id: I28fea4ff67ffed82465e5cfa785414069e4a180b
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>
Signed-off-by: Umang Agrawal <uagrawal@codeaurora.org>