fdtdec: dtb_dt_embedded: replace ifdefs by IS_ENABLED()

Patch fixes the checkpatch warnings like:
```
  WARNING: Use 'if (IS_ENABLED(CONFIG...))' instead of '#if or #ifdef'
  #94: FILE: lib/fdtdec.c:102:
  +#ifdef CONFIG_OF_EMBED
```

Signed-off-by: Evgeny Bachinin <EABachinin@salutedevices.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/fdtdec.c b/lib/fdtdec.c
index 9cb94a1..7f14188 100644
--- a/lib/fdtdec.c
+++ b/lib/fdtdec.c
@@ -99,15 +99,15 @@
 /* Get a pointer to the embedded devicetree, if there is one, else NULL */
 static u8 *dtb_dt_embedded(void)
 {
-#ifdef CONFIG_OF_EMBED
-# ifdef CONFIG_XPL_BUILD
-	return __dtb_dt_spl_begin;
-# else
-	return __dtb_dt_begin;
-# endif
-#else
-	return NULL;
-#endif
+	u8 *addr = NULL;
+
+	if (IS_ENABLED(CONFIG_OF_EMBED)) {
+		addr = __dtb_dt_begin;
+
+		if (IS_ENABLED(CONFIG_XPL_BUILD))
+			addr = __dtb_dt_spl_begin;
+	}
+	return addr;
 }
 
 const char *fdtdec_get_srcname(void)