tracing/filter: Define op as the enum that it is

The trace_events_file.c filter logic can be a bit complex. I copy this into
a userspace program where I can debug it a bit easier. One issue is the op
is defined in most places as an int instead of as an enum, and gdb just
gives the value when debugging. Having the actual op name shown in gdb is
more useful.

This has no functionality change, but helps in debugging when the file is
debugged in user space.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>