From: Bjorn Helgaas <bjorn.helgaas@hp.com>

uart_set_options() can dereference a null pointer.  This happens if you
specify a console that hasn't previously been setup by early_serial_setup().

For example, on ia64, the HCDP typically tells us about line 0, so we calls
early_serial_setup() for it.  If the user specifies "console=ttyS3", we
machine-check when trying to follow the uninitialized port->ops pointer.

It's not entirely clear to me whether we should return 0 or -ENODEV or
something.  The advantage of returning zero is that if the user specifies
"console=ttyS0" and we just lack the HCDP, the console doesn't work as early
as usual, but it does start working after the serial driver detects the port
(though the baud/parity/etc from the command line are lost).  Returning
-ENODEV seems to prevent it from ever working.