ASoC: stm32: sai: increase channels_max limit

The SAI peripheral supports up to 16 channels in TDM mode (8L+8R).
The driver currently supports TDM over two channels.
Increase SAI DAI playback/record channels_max,
to also allow up to 16 channels in TDM mode.

Signed-off-by: Olivier Moysan <olivier.moysan@st.com>
Link: https://lore.kernel.org/r/20211117110031.19345-1-olivier.moysan@foss.st.com
Signed-off-by: Mark Brown <broonie@kernel.org>