Fix warning injected into user code by @chiselName (#2500) (#2503)

In Scala 2.13, Auto-application to `()` is deprecated. Any nullary
method (ie. a method with no arguments) that is defined with () must now
be called with (). @chiselName used to inject a case of this warning
into user code which would then cause a warning on the @chiselName macro
that the user has no way to fix.

(cherry picked from commit ca902680

)

Co-authored-by: Jack Koenig <koenig@sifive.com>