ASoC: stm32: sai: add restriction on mmap support

Do not support mmap in S/PDIF mode. In S/PDIF mode
the buffer has to be copied, to allow the channel status
bits insertion.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Link: https://lore.kernel.org/r/20191104133654.28750-1-olivier.moysan@st.com


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