staging: greybus: Added do - while in multi statement macro

This patch add fixes an checkpatch error for "Macros with multiple statements
should be enclosed in a do - while loop"

Signed-off-by: Hemansh Agnihotri <hemanshagnihotri27@gmail.com>

drivers/staging/greybus/loopback.c

@@ -162,10 +162,11 @@ static ssize_t name##_avg_show(struct device *dev,		\
 }									\
 static DEVICE_ATTR_RO(name##_avg)
 
-#define gb_loopback_stats_attrs(field)				\
+#define gb_loopback_stats_attrs(field) do { \
 	gb_loopback_ro_stats_attr(field, min, u);		\
 	gb_loopback_ro_stats_attr(field, max, u);		\
-	gb_loopback_ro_avg_attr(field)
+	gb_loopback_ro_avg_attr(field);				\
+	} while (0)
 
 #define gb_loopback_attr(field, type)					\
 static ssize_t field##_show(struct device *dev,				\