serial: samsung: check DMA engine capabilities before using DMA mode

DMA engine driver might not always provide all the features needed by
serial driver to properly operate in DMA mode, so check that before
selecting DMA mode.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>