dmaengine: coh901318: return ssize_t for coh901318_debugfs_read

The file_operations.read expects return as ssize_t, so update
coh901318_debugfs_read to return ssize_t to fix the warning:

drivers/dma/coh901318.c:1369:2: warning: initialization from incompatible pointer type [enabled by default]
  .read  = coh901318_debugfs_read,

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>