staging:iio:dac:ad5446: Fix 24bit transfers

We currently only write 16 bit in case where we should write 24 bit. The spi message
length is calculated from the channel storage_size, but since the storage size
is only 16 bit we end up with the wrong value for devices which have power down
bits and thus a register with 24 bit. Since each store function knows how many
bytes it has to write just use the spi_write function from there instead of
going through the hassle of manually preparing a spi_message and keeping buffers
in the state struct.

Another advantage of this patch is that it will make implementing support for
similar I2C based DACs much easier.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>