dma: fix fsldma build warnings

drivers/dma/fsldma.c: In function 'fsldma_cleanup_descriptor':
drivers/dma/fsldma.c:860:6: warning: unused variable 'len' [-Wunused-variable]
drivers/dma/fsldma.c:859:13: warning: unused variable 'dst' [-Wunused-variable]
drivers/dma/fsldma.c:858:13: warning: unused variable 'src' [-Wunused-variable]
drivers/dma/fsldma.c:857:17: warning: unused variable 'dev' [-Wunused-variable]
- due to unmap changes

drivers/dma/fsldma.c: In function 'fsl_dma_tx_submit':
drivers/dma/fsldma.c:428:2: warning: 'cookie' may be used uninitialized in this function [-Wuninitialized]
- long standing warning

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Li Yang <leoli@freescale.com>
Cc: Zhang Wei <zw@zh-kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>