imx: serial_mxc: use CONFIG_IS_ENABLED instead of ifdef

Kconfig allows boards to configured with DM_SERIAL and still
have SPL_DM_SERIAL disabled.  This patch changes the ifdef's
to CONFIG_IS_ENABLED to allow the modes to differ between
SPL and U-Boot.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/drivers/serial/serial_mxc.c b/drivers/serial/serial_mxc.c
index df35ac9..476df25 100644
--- a/drivers/serial/serial_mxc.c
+++ b/drivers/serial/serial_mxc.c
@@ -178,7 +178,7 @@
 	writel(UCR1_UARTEN, &base->cr1);
 }
 
-#ifndef CONFIG_DM_SERIAL
+#if !CONFIG_IS_ENABLED(DM_SERIAL)
 
 #ifndef CONFIG_MXC_UART_BASE
 #error "define CONFIG_MXC_UART_BASE to use the MXC UART driver"
@@ -260,7 +260,7 @@
 }
 #endif
 
-#ifdef CONFIG_DM_SERIAL
+#if CONFIG_IS_ENABLED(DM_SERIAL)
 
 int mxc_serial_setbrg(struct udevice *dev, int baudrate)
 {