power: qpnp-linear-charger: Force VREF_THERM to always-on

Always keep VREF_BAT_THM forced-on. In active state
it is required for battery therm/presence detection and
in sleep it is required by BUA (battery UICC alarm).

Change-Id: I2e11381ec9a1027f600f95066aaad681443f338b
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>