serial: zynq: Use static inline for _debug_uart_init()

Mark _debug_uart_init() as static to avoid sparse warning and
inline it to debug_uart_init().

Reported-by: Thomas Chou <thomas@wytron.com.tw>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Thomas Chou <thomas@wytron.com.tw>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>

drivers/serial/serial_zynq.c

@@ -192,7 +192,7 @@ U_BOOT_DRIVER(serial_zynq) = {
 };
 
 #ifdef CONFIG_DEBUG_UART_ZYNQ
-void _debug_uart_init(void)
+static inline void _debug_uart_init(void)
 {
 	struct uart_zynq *regs = (struct uart_zynq *)CONFIG_DEBUG_UART_BASE;