thermal: tsens: Enable valid status bit check

Check if the valid bit is set as part of reading the ADC data from the
status register. The valid bit is used to avoid possibility of reading
invalid ADC data as the controller is updating the readings.

Change-Id: Ice14075a3444d758b981cb0b43ea46860b5dc3cf
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>