charger-manager: Fix bug when check dropped voltage after fullbatt event

This patch check difference value between current voltage of battery and
desc->fullbatt_uV whether positve or negative number. If difference value
is negative number when current voltage of battery is larger than
desc->fullbatt_uV, charger-manager return immediately because battery is
fully charged.

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>