MPC52xx_PSC_IMR_TXRDY
port->read_status_mask |= MPC52xx_PSC_IMR_RXRDY | MPC52xx_PSC_IMR_TXRDY;
& MPC52xx_PSC_IMR_TXRDY;
port->read_status_mask |= MPC52xx_PSC_IMR_TXRDY;
port->read_status_mask &= ~MPC52xx_PSC_IMR_TXRDY;