dmaengine: stm32-dmamux: use reset controller only at probe time

Remove reset controller reference from device instance since it is
used only at probe time.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>
Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/20200128094158.20361-4-amelie.delaunay@st.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>