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>