serial: samsung: Simplify DMA engine initialization code

dma_request_slave_channel_compat() requires filter function and mask, which
are not needed on device tree based platforms, so simplify the code by
calling the more appropriate dma_request_chan() function. This additionally
gives us proper error handling, because the new function returns error
codes instead of NULL on failure.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>