thermal: qpnp-adc-tm: Keep VADC_TM peripheral always enabled

After a client issues a threshold monitoring request and
once the thresholds are enabled on the ADC_TM there are
cases where the client can request to disable the ADC_TM
requests. If there are other conversions taking place
on other VADC peripherals this may cause the data from the
ADC_TM peripheral to be latched to the next request from
other VADC peripheral leading to incorrect results. Therefore
keep the ADC_TM peripheral enabled.

Change-Id: I4415e5bf5430667057588694f3146c4355968428
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>