Input: ads7846 - select correct SPI mode

Talk to ADS7846 chip using SPI mode 1, which is what the chip
supports: writes on falling clock edge, reads on rising.

Signed-off-by: Imre Deak <imre.deak@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>