tracing: remove redundant SOFTIRQ from softirq event traces

After converting the softirq tracer to use te flags options, this
caused a regression with the name. Since the flag was used directly
it was printed out (i.e. HRTIMER_SOFTIRQ).

This patch only shows the softirq name without the SOFTIRQ part.

[ Impact: fix regression of output from softirq events ]

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