drm/sun4i: sun6i_mipi_dsi: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: multi_v7_defconfig arm):

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c: In function ‘sun6i_dsi_transfer’:
drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c:993:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
if (msg->rx_len == 1) {
^
drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c:998:2: note: here
default:
^~~~~~~

Signed-off-by: Gustavo A. R. Silva <[e-mail]>

drivers/gpu/drm/sun4i/sun6i_mipi_dsi.c

···
 			ret = sun6i_dsi_dcs_read(dsi, msg);
 			break;
 		}
+		/* Else, fall through */
 
 	default:
 		ret = -EINVAL;