serial: 8250_dma: no need to sync RX buffer

RX buffer is allocated from DMA coherent memory. Thus there is no need to call
DMA sync API for it.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>