8250: mediatek: mark PM functions as __maybe_unused

The mtk8250_runtime_suspend function is not used when runtime PM is
disabled, so we get a warning about an unused function:

drivers/tty/serial/8250/8250_mtk.c:119:12: error: 'mtk8250_runtime_suspend' defined but not used [-Werror=unused-function]
 static int mtk8250_runtime_suspend(struct device *dev)

This marks all the PM functions as __maybe_unused to avoid the warning,
and removes the #ifdef around the PM_SLEEP functions for consistency.

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