power: batterydata-lib: Use minimum slope if its value is <= 0

The OCV vs SOC slope cannot be <= 0. The VMBMS uses it for SOC
calculation and a zero slope can keep the SOC stuck at a value.

Fix this by reporting a minimum slope if its <= 0.

Change-Id: I8a4695c0ebf8200c96364598abe23e7ac3c401bd
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>