tracing: Fix return value of tracing_stats_read()

The function tracing_stats_read() mistakenly returns ENOMEM instead
of the negative value -ENOMEM.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
LKML-Reference: <4AFB2C0B.50605@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>