[media] media: ti-vpe: vpdma: fix a timeout loop

The check assumes that we end on zero but actually we end on -1.  Change
the post-op to a pre-op so that we do end on zero.  Techinically now we
only loop 499 times instead of 500 but that's fine.

Fixes: dc12b124 ("[media] media: ti-vpe: vpdma: Add abort channel desc and cleanup APIs")

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Benoit Parrot <bparrot@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>