tty: serial: imx: fix lockup in rs485 after close
Fixes an issue when the RS485 interface is closed in situations when transmitter shift register is not yet empty. After closing the RS485 interface in this situation the interface is no more able to transmit data in RS485 mode, even on subsequent open and close actions.
The reason for this is, that tx_state is left in SEND state on close when the transmitter shift register is not yet empty. So the next open of the device will find the tx_state in SEND state and therefore will not transmit any data.