power: smb358-charger: Query VADC only if the channel is supported

On different SMB358 + PMIC designs the VBAT/BATT_THERM may/may-not
be routed to the PMIC. Enable access to these ADC channels only if
they are enabled.

Change-Id: I4c21ed0f760cc9e8f685b8e5695c3ba354a00209
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>