ASoC: stm32: Add full duplex support to i2s

This patch allows to use i2s interface either as single
audio path (rx or tx), or bidirectional audio path.
This patch is added separately, as the driver does not
follow recommended use of the interface, to support this
configuration.

Signed-off-by: olivier moysan <olivier.moysan@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>