drm/omap: dsi: fix unsigned expression compared with zero

r is "u32" always >= 0,mipi_dsi_create_packet may return little than zero.
so r < 0 condition is never accessible.

Fixes coccicheck warnings:
./drivers/gpu/drm/omapdrm/dss/dsi.c:2155:5-6:
WARNING: Unsigned expression compared with zero: r < 0

Signed-off-by: Junlin Yang <yangjunlin@yulong.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210312071445.1721-1-angkery@163.com