swiotlb: Convert swiotlb_force from int to enum

Convert the flag swiotlb_force from an int to an enum, to prepare for
the advent of more possible values.

Suggested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
index 183f37c..71d104e4 100644
--- a/include/linux/swiotlb.h
+++ b/include/linux/swiotlb.h
@@ -9,7 +9,12 @@
 struct page;
 struct scatterlist;
 
-extern int swiotlb_force;
+enum swiotlb_force {
+	SWIOTLB_NORMAL,		/* Default - depending on HW DMA mask etc. */
+	SWIOTLB_FORCE,		/* swiotlb=force */
+};
+
+extern enum swiotlb_force swiotlb_force;
 
 /*
  * Maximum allowable number of contiguous slabs to map,