serial: 8250_mtk: drop !defined(MODULE) conditional

The !defined(MODULE) conditional has been added to the
OF_EARLYCON_DECLARE() define.

This commit partially reverts commit 3f5921a6

 ("serial:
8250/mediatek: fix building with SERIAL_8250=m").

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>