Locking: port_sem taken.
Interrupts: caller dependent.
+ flush_buffer(port)
+ Flush any write buffers, reset any DMA state and stop any
+ ongoing DMA transfers.
+
+ This will be called whenever the port->info->xmit circular
+ buffer is cleared.
+
+ Locking: port->lock taken.
+ Interrupts: locally disabled.
+ This call must not sleep
+
set_termios(port,termios,oldtermios)
Change the port parameters, including word length, parity, stop
bits. Update read_status_mask and ignore_status_mask to indicate