thermal: qpnp-adc: Update VADC2_TM disable routine

After disabling the peripheral, the STS_REQ bit is checked
to ensure there are no more pending transactions in queue.
However there are instance when the STS_REQ bit does not
fall low since the peripheral is disabled. This is in
conjunction with a clock gating feature that was added
to the 8226 PMIC. Therefore as a workaround when disabling
the peripheral switch the mode to a single conversion mode
and check if the STS_REQ bit falls low. Ensure that when
disabling the corresponding channel the MULTI_MEASUREMENT_ENABLE
bit for that channel stays high when the above check is being
performed.

Change-Id: I4c234cfb273952a99854ccd07859f8d24d0b3fb7
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>