dix: check_modmap_change() returns Success, not true

Not sure what if anything calls XSetDeviceModifierMapping() but this would've
failed all the time. check_modmap_change() returns Success but we were
treating it like a boolean. Fix this.

Reported-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Keith Packard <keithp@keithp.com>