s390/traps: do not test MONITOR CALL without CONFIG_BUG

tinyconfig fails to boot, because without CONFIG_BUG report_bug()
always returns BUG_TRAP_TYPE_BUG, which causes mc 0,0 in
test_monitor_call() to panic. Fix by skipping the test without
CONFIG_BUG.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>