serial: omap8250: Terminate rx dma only for flushes

DMA completed normally does not require termination; only terminate
paused rx dma stemming from rx dma flush.

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