video: stm32: Fix not calling dev_xxx with a device

There is no member `dev` in dw_mipi_dsi, but there is one in mipi_dsi_host,
so use that.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@st.com>
Tested-by: Patrick Delaunay <patrick.delaunay@st.com>