staging: greybus: eclose macro in a do - while loop

" ERROR: Macros with multiple statements should be enclosed in a do -
while loop"

Reported by checkpath.

do loop with the conditional expression set to a constant
value of zero (0).This creates a loop that
will execute exactly one time.This is a coding idiom that
allows a multi-line macro to be used anywhere
that a single statement can be used.

So, enclose `gb_loopback_stats_attrs` macro in do - while (0) to
fix checkpath error

Signed-off-by: Menna Mahmoud <eng.mennamahmoud.mm@gmail.com>

drivers/staging/greybus/loopback.c

@@ -162,10 +162,12 @@ static ssize_t name##_avg_show(struct device *dev,		\
 }									\
 static DEVICE_ATTR_RO(name##_avg)
 
-#define gb_loopback_stats_attrs(field)				\
-	gb_loopback_ro_stats_attr(field, min, u);		\
-	gb_loopback_ro_stats_attr(field, max, u);		\
-	gb_loopback_ro_avg_attr(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);				\
+	} while (0)
 
 #define gb_loopback_attr(field, type)					\
 static ssize_t field##_show(struct device *dev,				\