perf tui: Fix last use_browser problem related to .perfconfig

When we moved to using ~/.perfconfig to set the value of use_browser,
it changed from a boolean to an int so that the convention used for
use_pager was followed.

That convention is:

-1: unspecified, that is what use_{browser,pager} is initialized
 0: Don't use the browser (should be TUI), because was explicitely
    set to 0/off/false on ~/.perfconfig [tui] cmd =, or because
    we're redirecting the stdout to a file or piping it to some
    other command (!isatty()).
 1: Use the TUI

Some code was not properly audited and continued testing it as a
boolean, this seems to be the last one.

Reported-by: Frédéric Weisbecker <fweisbec@gmail.com>
Tested-by: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Cc: Tom Zanussi <tzanussi@gmail.com>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>