staging: kpc2000: double unlock in error handling in kpc_dma_transfer()

The goto err_descr_too_many; calls unlock_engine() so this unlocks
twice.

Fixes: 7df95299

 ("staging: kpc2000: Add DMA driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>