Input: ad714x - use DMA-safe buffers for spi_write()

spi_write() requires use of DMA-safe (cacheline aligned) buffers.
Also use the same buffers when reading data since to avoid extra
locking and potential memory allocation in spi_write_then_read().

Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>