serial: 8250_dw: provide a filter for DMA channel detection

The channel IDs are set to -1 by default. It will prevent
dmaengine from trying to provide the first free channel if
it fails to allocate exclusive channel. This will fix an
issue with ACPI enumerated UARTs that do not support DMA
but still end up getting a DMA channel incorrectly.

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