can: peak_canfd: 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 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

---
 drivers/net/can/peak_canfd/peak_pciefd_main.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

index d08a3d559114..6ad83a881039 100644
@@ -146,7 +146,7 @@ struct pciefd_rx_dma {
 	__le32 irq_status;
 	__le32 sys_time_low;
 	__le32 sys_time_high;
-	struct pucan_rx_msg msg[0];
+	struct pucan_rx_msg msg[];
 } __packed __aligned(4);
 
 /* Tx Link record */

@@ -194,7 +194,7 @@ struct pciefd_board {
 	struct pci_dev *pci_dev;
 	int can_count;
 	spinlock_t cmd_lock;		/* 64-bits cmds must be atomic */
-	struct pciefd_can *can[0];	/* array of network devices */
+	struct pciefd_can *can[];	/* array of network devices */
 };
 
 /* supported device ids. */

2.25.0