power: step-chg-jeita: update jeita/step ranges to support signed threshold

Currently software JEITA uses unsigned variable to store threshold data
and thus fails to support negative temperature ranges, fix this by using
signed variable to store threshold data.

Change-Id: Ia888610e9e4ee4606333a07a3f338a494ec0392e
Signed-off-by: Ashay Jaiswal <ashayj@codeaurora.org>