tracing/function: Convert func_set_flag() to a switch statement

Since the function accepts just one bit, we can use the switch
construction instead of if/else if/...

Just a cosmetic change, there should be no functional changes.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>