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