tty/serial: omap: empty the RX FIFO at the end of half-duplex TX

Provided that the SER_RS485_RX_DURING_TX flag is not set, empty the
RX FIFO to prevent reading back the transmitted data.

Signed-off-by: Dimitris Lampridis <dlampridis@logikonlabs.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>