altera_uart: Adjust the declaration of debug_uart_init()

Follow commit 97b05973 ("debug_uart: Adjust the declaration of
debug_uart_init()")

Signed-off-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Marek Vasut <marex@denx.de>
Reviewed-by: Chin Liang See <clsee@altera.com>