Draft: serial:imx: Busywait for RS485 half duplex to reduce turnarround time

The driver rescheduled the 'imx_uart_stop_tx' function twice before the TX enable signal is disabled. This introduces some variance in the turnarround time. Adding a busywait boefore calling the stop reduces this. Also the timer for delay_before_rts is not started when the delay is 0 now.