dmaengine: edma: avoid uninitialized variable use

dmaengine: edma: avoid uninitialized variable use

If edma_read_slot() gets an invalid argument, it does not set a result,
as found by "gcc -Wmaybe-uninitialized"

drivers/dma/edma.c: In function 'dma_ccerr_handler':
drivers/dma/edma.c:1499:21: error: 'p.a_b_cnt' may be used uninitialized in this function [-Werror=maybe-uninitialized]
drivers/dma/edma.c:1499:21: error: 'p.ccnt' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  if (p.a_b_cnt == 0 && p.ccnt == 0) {

If we change the function to return an error in this case, we can handle
the failure more gracefully and treat this the same way as a null slot
that we already catch.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>