tracing/filter: Correct potential uninitialized variable

'ret' in __ftrace_function_set_filter() may be used uninitialized
if 're_count' is zero. Fix this to avoid a compiler warning.

Change-Id: I0a257159141d86d92573c28d233a3653e89c48ea
Signed-off-by: Matt Wagantall <mattw@codeaurora.org>