ASoC: stm32: dfsdm: fix 16 bits record

In stm32_afsdm_pcm_cb function, the transfer size is provided in bytes.
However, samples are copied as 16 bits words from iio buffer.
Divide by two the transfer size, to copy the right number of samples.

Fixes: 1e7f6e1c ("ASoC: stm32: dfsdm: add 16 bits audio record support")

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Link: https://lore.kernel.org/r/20200110131131.3191-1-olivier.moysan@st.com


Signed-off-by: Mark Brown <broonie@kernel.org>