serial: 8250: 8250_omap: Account for data in flight during DMA teardown

Take into account data stuck in DMA internal buffers before pushing data
to higher layer. dma_tx_state has "in_flight_bytes" member that provides
this information.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Link: https://lore.kernel.org/r/20200319110344.21348-3-vigneshr@ti.com


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