drm/dsi: Make mipi_dsi_dcs_write() return ssize_t

This function returns the value of the struct mipi_dsi_host_ops'
.transfer() so make sure the return types are consistent.

Acked-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>