spi: s3c64xx: remove unnecessary callback msg->complete

msg->complete will be called in spi_finalize_current_message().

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>