power: qpnp-charger: remove module parameter vadc reads

The qpnp-vadc driver will require reference pointers
for vadc reads in the future. Since there is no visibility
into the device tree from module parameters which act like
globals remove these and expose these readings correctly
via the power supply framework in the future.

Change-Id: I32affa84b1af5f9a5520b4d47db64a2307cb843f
Signed-off-by: David Keitel <dkeitel@codeaurora.org>