USB: use function attribute __maybe_unused

Substitute USB instances of __attribute__ ((unused)) functions with the
newly introduced __maybe_unused.

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>