tracing/filters: Fix to make system filter work

commit fce29d15b59245597f7f320db4a9f2be0f5fb512
("tracing/filters: Refactor subsystem filter code")
broke system filter accidentally.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <4AE810BD.3070009@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>