The driver was treating -EBUSY as indication of queueing to backlog
without checking that backlog is enabled for the request.

Fix it by checking request flags.
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Vic Wu <vic.wu@mediatek.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>