Input: ads7846 - make transfer buffers DMA safe

req.sample needs its own cacheline otherwise accessing req.msg fetches
it in again.

Note: This effect doesn't occur if the underlying SPI driver doesn't use
DMA at all.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>