tracing: initialize return value for __ftrace_set_clr_event

Commit 8f31bfe5
tracing/events: clean up for ftrace_set_clr_event()

Moved out the code for ftrace_set_clr_event into a helper funciton but
did not initialize the return value. As a result, we do not warn about
a typo in the echoing of events in set_event.

This patch restores the old warning:

 # echo foobar > set_event
-bash: echo: write error: Invalid argument

[ Impact: restore warning of invalid entries to set_event ]

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>