ads7846: convert to input_allocate_device

Patch from David Brownell and Dmitry Torokhov: convert the ads7846 driver
to use input_allocate_device. Also fixes the bug where the SPI message was
not initialized with spi_message_init().

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