serial: bfin_5xx: disable CON_PRINTBUFFER for consoles

If we are using early serial, don't let the normal console rewind
the log buffer, since that causes things to be printed multiple times.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>