Submit a seperate request for powering down the ADC in ads7846,
doing it after the last read request.  Otherwise some of the read
values are incorrect.
Signed-off-by: NImre Deak <imre.deak@nokia.com>
Signed-off-by: NJuha Yrjola <juha.yrjola@nokia.com>
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NDmitry Torokhov <dtor@mail.ru>