dmaengine: s3c24xx: missing unlock on an error path

We should unlock here before returning -EINVAL.

Fixes: 39ad4600 ('dmaengine: s3c24xx: Split device_control')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>