tty: serial/8250: remove console dependency for mediatek

If the mediatek serial port driver is built-in, but serial
console is disabled in Kconfig (e.g. when the serial driver
itself is a loadable module), we get this build error:

drivers/built-in.o: In function `early_mtk8250_setup':
undefined reference to `early_serial8250_setup'

To avoid that problem, this patch encloses the early_mtk8250_setup
function in #ifdef CONFIG_SERIAL_8250_CONSOLE, the same symbol
that guards the early_serial8250_setup function.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Eddie Huang <eddie.huang@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>