serial: 8250_dma: Use dmaengine helpers to get the slave channels

The helper functions in dmaengine API allow the drivers to
request slave channels without the filter parameters. They
will attempt to extract the needed DMA client information
from DT or ACPI, but if such information is not available
the filter parameters can still be used.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>