tty: serial: 8250: add CON_CONSDEV to flags

When using the 8250 as a boot console and the main console results in
messages being printed twice.  The console framework will only
unregister boot consoles if a new console is registered with the
CON_CONSDEV flag set.

Set this flag for the univ8250 console to prevent double-registration.

Signed-off-by: Matthew Leach <matthew.leach@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>