perf hists browser: Do not exit when 'f' is pressed in 'report' mode

perf hists browser: Do not exit when 'f' is pressed in 'report' mode

The 'f' hotkey is only used when in 'top', dynamic mode, to
enable/disable events, currently not making sense in the 'report',
static mode, where we can't go from showing the histogram entries
created from a perf.data file to adding more events after recreating the
evlist created from the perf.data file, albeit possible, this is not
implemented right now.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-lholzf472pu98dkkijggwx2m@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>