iio: qcom-tadc: use HW default for batt therm HW conversion trigger

The battery thermistor channel is unused in parallel charge applications
since the primary charger disables the parallel charger when hard JEITA
limits are reached.

In standalone applications the battery thermistor channel is used since
it is required for JEITA.

The hardware defaults take care of both of these applications hence
mask the register writes to enable only the die temperature, and
connector thermistor hardware conversion triggers. This will ensure the
battery thermistor hardware conversion trigger is left to the hardware
default.

Change-Id: Iea2fc779562436dfae3bd41c944d5727366006b2
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>