regmap: fix the warning about unused variable

The variable 'u64 *u64' should be only visible on 64-BIT platform.

Signed-off-by: Xiubo Li <lixiubo@cmss.chinamobile.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
diff --git a/drivers/base/regmap/regmap.c b/drivers/base/regmap/regmap.c
index 1791180..a0d30a0 100644
--- a/drivers/base/regmap/regmap.c
+++ b/drivers/base/regmap/regmap.c
@@ -2581,7 +2581,9 @@
 				 * we assume that the values are native
 				 * endian.
 				 */
+#ifdef CONFIG_64BIT
 				u64 *u64 = val;
+#endif
 				u32 *u32 = val;
 				u16 *u16 = val;
 				u8 *u8 = val;