drm/omap: dsi: fix unreachable code in dsi_vc_send_short()

The 'r' in dsi_vc_send_short() is of type 'unsigned int', so the
'r < 0' can't be true.

Fix this by introducing a 'err' of type 'int' insteaded.

Fixes: 1ed62538

 ("drm/omap: dsi: switch dsi_vc_send_long/short to mipi_dsi_msg")

Signed-off-by: Menglong Dong <dong.menglong@zte.com.cn>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210127015117.23267-1-dong.menglong@zte.com.cn