tty: serial: msm_serial: Use dma_request_chan() directly for channel request

dma_request_slave_channel_reason() is:
#define dma_request_slave_channel_reason(dev, name) \
	dma_request_chan(dev, name)

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Link: https://lore.kernel.org/r/20191113094618.1725-3-peter.ujfalusi@ti.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>