ASoC: compress: Only assign compr->ops->copy once

There are only one set of ops on the compressed stream so no need to
reassign the copy callback repeatedly, stop after copy is seen to be
necessary.

Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>