tracing: fix for tracepoint API change

Commit 38516ab5 ("tracing: Let
tracepoints have data passed to tracepoint callbacks") requires this
fixup to the powerpc code.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>