Input: st1232 - note that the receive buffer is DMA-safe

The receiving buffer is allocated separately from the main driver
data structure, and is naturally DMA-safe, so mark it as such when
building I2C transfer message.

Tested-by: Matthias Fend <Matthias.Fend@wolfvision.net>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>