ftrace: Fix the conditional that updates $ref_func

Fix the conditional that checks if we already have a $ref_func and that
the new function is weak. The code as previously checking whether either
condition was false, and we really need to only update $ref_func is both
cconditions are false.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
LKML-Reference: <1248365775-25196-1-git-send-email-matt@console-pimps.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>