ASoC: stm32: sai: add restriction on mmap support

ASoC: stm32: sai: add restriction on mmap support

BugLink: https://bugs.launchpad.net/bugs/1855306
[ Upstream commit eaf072e512d54c95b0977eda06cbca3151ace1e5 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>