drm/bridge/synopsys: dsi: Fix dsi_host_transfer() return value

The dw_mipi_dsi_host_transfer() must return the number of
bytes transmitted/received on success instead of 0.
Note: As the read feature is not implemented, only the
transmitted number of bytes is returned for the moment.

Reviewed-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180125103800.1999-3-philippe.cornu@st.com