ads7846: fix ADC power down bug

Submit a seperate read request for powering down the ADC in ads7846,
instead of doing it in the last read request. Otherwise some of the read
values are incorrect.

Signed-off-by: Imre Deak <imre.deak@nokia.com>
Signed-off-by: Juha Yrjölä <juha.yrjola@nokia.com>