serial: 8250_early: Assume uart already initialized if no baud option

The <baud><parity><bit> option string is not supplied if the earlycon
is started via devicetree and OF_EARLYCON_DECLARE(). The option string
is also not required if started via kernel command line parameters of
the form:
  earlycon=uart,mmio,<addr>
  console=uart,mmio,<addr>

If earlycon_device->baud is 0, then an option string was not supplied.
In this case, assume the uart has already been initialized by the
bootloader or firmware.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>