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

There is a regular need in the kernel to provide a way to declare having a
dynamically sized set of trailing elements in a structure. Kernel code should
always use “flexible array members”[1] for these cases. The older style of
one-element or zero-length arrays should no longer be used[2].

[1] https://en.wikipedia.org/wiki/Flexible_array_member
[2] https://www.kernel.org/doc/html/v5.9-rc1/process/deprecated.html#zero-length-and-one-element-arraysSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/net/ethernet/freescale/enetc/enetc_qos.c

@@ -402,7 +402,7 @@ struct enetc_psfp_gate {
 	u32 num_entries;
 	refcount_t refcount;
 	struct hlist_node node;
-	struct action_gate_entry entries[0];
+	struct action_gate_entry entries[];
 };
 
 /* Only enable the green color frame now