rpmsg: Constify local variable in field store macro

Memory pointed by variable 'old' in field store macro is not modified,
so it can be made a pointer to const.
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220419113435.246203-12-krzysztof.kozlowski@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/rpmsg/rpmsg_core.c

@@ -400,7 +400,8 @@ field##_store(struct device *dev, struct device_attribute *attr,	\
 	      const char *buf, size_t sz)				\
 {									\
 	struct rpmsg_device *rpdev = to_rpmsg_device(dev);		\
-	char *new, *old;						\
+	const char *old;						\
+	char *new;							\
 									\
 	new = kstrndup(buf, sz, GFP_KERNEL);				\
 	if (!new)							\