tracing: Fix return value of ftrace_raw_output_prep()

If the trace_seq of ftrace_raw_output_prep() is full this function
returns TRACE_TYPE_PARTIAL_LINE, otherwise it returns zero.

The problem is that TRACE_TYPE_PARTIAL_LINE happens to be zero!

The thing is, the caller of ftrace_raw_output_prep() expects a
success to be zero. Change that to expect it to be
TRACE_TYPE_HANDLED.

Link: http://lkml.kernel.org/r/20141114112522.GA2988@dhcp128.suse.cz

Reminded-by: Petr Mladek <pmladek@suse.cz>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>