tracing: Convert pr_warning() to pr_warn() in trace_events.c

Convert pr_warning to standard pr_warn
Define pr_fmt(fmt) fmt to avoid any future default fmt definition

Link: http://lkml.kernel.org/p/1402141388-21144-1-git-send-email-fabf@skynet.be



Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>