power: qpnp-charger: fix power stage work around

For power stage work around, it uses prev_usb_max_ma
as the judgement for whether need reduce power stage.
We should guarantee this variable be initialized before
reduce power stage work around scheduled.

Change-Id: Ide00b540a0a26e5593a9b924fbd329c4ca9b20f6
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>