power: qpnp-smbcharger: fix vbus oscillations

When charging at high currents, the charger buck instabilities can cause
large oscillations on vbus (USB voltage). These oscillations in turn
cause many different other issues, such as large swings in battery current
readings.

Work around this problem by increasing the switching rate of the charger
buck, so that the wave form smoothens out.

CRs-Fixed: 710498
Change-Id: I80bc158eab8051c6a31ad14f08d541fe59c0744c
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>