Input: ads7846 - power down ADC a bit later

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: Imre Deak <imre.deak@nokia.com>
Signed-off-by: Juha Yrjola <juha.yrjola@nokia.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>