sf: Use const for the stage name

This is not updated at runtime so should be marked const. Update the code
accordingly.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

cmd/sf.c

@@ -394,7 +394,7 @@ enum {
 	STAGE_COUNT,
 };
 
-static char *stage_name[STAGE_COUNT] = {
+static const char *stage_name[STAGE_COUNT] = {
 	"erase",
 	"check",
 	"write",