serial: omap8250: Drop rx buffer sync

commit 27c310c5 ("serial: 8250_dma: no need to sync RX buffer")
notes the RX DMA buffer is allocated from DMA coherent memory, and
thus does not need sync'd for each transaction. The same is true
for OMAP RX DMA.

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>