dmatest: check for dma mapping error

The kernel emits a warning if CONFIG_DMA_API_DEBUG=y:

  WARNING: at lib/dma-debug.c:933 check_unmap+0x5d6/0x6ac()
  dw_dmac dw_dmac.0: DMA-API: device driver failed to check map error[device address=0x0000000035698305] [size=14365 bytes] [mapped as single]

Fix this by adding the required checking of the dma_map_single() return
value.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Cc: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>