serial: bfin_5xx: kgdboc should accept gdb break only when it is active

If we always check for gdb breaks even when it isn't active, we get false
positives on normal code and the system panics.

URL: http://blackfin.uclinux.org/gf/tracker/5277



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>