freescale: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language
extension to the C90 standard, but the preferred mechanism to declare
variable-length types such as these ones is a flexible array member[1][2],
introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning
in case the flexible array does not occur last in the structure, which
will help us prevent some kind of undefined behavior bugs from being
inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by
this change:

"Flexible array members have incomplete type, and so the sizeof operator
may not be applied. As a quirk of the original implementation of
zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 7649773293 ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/freescale/enetc/enetc.h

@@ -163,7 +163,7 @@ struct enetc_int_vector {
 	char name[ENETC_INT_NAME_MAX];
 
 	struct enetc_bdr rx_ring ____cacheline_aligned_in_smp;
-	struct enetc_bdr tx_ring[0];
+	struct enetc_bdr tx_ring[];
 };
 
 struct enetc_cls_rule {

drivers/net/ethernet/freescale/enetc/enetc_hw.h

@@ -588,7 +588,7 @@ struct tgs_gcl_data {
 	__le32		bth;
 	__le32		ct;
 	__le32		cte;
-	struct gce	entry[0];
+	struct gce	entry[];
 };
 
 struct enetc_cbd {

drivers/net/ethernet/freescale/fec.h

@@ -584,7 +584,7 @@ struct fec_enet_private {
 	int pps_enable;
 	unsigned int next_counter;
 
-	u64 ethtool_stats[0];
+	u64 ethtool_stats[];
 };
 
 void fec_ptp_init(struct platform_device *pdev, int irq_idx);