Correct SPL use of DEBUG_UART_ZYNQ

This converts 1 usage of this option to the non-SPL form, since there is
no SPL_DEBUG_UART_ZYNQ defined in Kconfig

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/serial_zynq.c b/drivers/serial/serial_zynq.c
index 4b18183..9bb9b7d 100644
--- a/drivers/serial/serial_zynq.c
+++ b/drivers/serial/serial_zynq.c
@@ -108,7 +108,7 @@
 
 static int _uart_zynq_serial_putc(struct uart_zynq *regs, const char c)
 {
-	if (CONFIG_IS_ENABLED(DEBUG_UART_ZYNQ)) {
+	if (IS_ENABLED(CONFIG_DEBUG_UART_ZYNQ)) {
 		if (!(readl(&regs->channel_sts) & ZYNQ_UART_SR_TXEMPTY))
 			return -EAGAIN;
 	} else {