power: qpnp-charger: fix power stage work around

When AICL support was added, the power stage work around for the
PM8026 coupling issue was broken. If the phone was booted with the USB
attached, the charger monitor will not have been loaded yet, so iusbmax
will be set to the default. This will in turn stop the power stage
reduction workaround even when the charger is connected to a dedicated
charging port.

Fix this by distinguishing between a target max usb current and the
set usb current. Use the target max usb current to determine whether the
work around is needed.

Change-Id: I4c1cab030a0164dfb3410a9a023ba98b3b90ad6e
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>