acpi-dma: convert to return error code when asked for channel

Currently acpi_dma_request_slave_chan_by_index() and
acpi_dma_request_slave_chan_by_name() return only requested channel or NULL.
This patch converts them to return appropriate error code instead of NULL in
case of unsuccessfull request.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>