Input: tca6416-keypad - 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 76497732 ("cxgb3/l2t: Fix undefined behaviour")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://lore.kernel.org/r/20200214172022.GA27490@embeddedorSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

drivers/input/keyboard/tca6416-keypad.c

@@ -33,7 +33,7 @@ MODULE_DEVICE_TABLE(i2c, tca6416_id);
 
 struct tca6416_drv_data {
 	struct input_dev *input;
-	struct tca6416_button data[0];
+	struct tca6416_button data[];
 };
 
 struct tca6416_keypad_chip {
@@ -48,7 +48,7 @@ struct tca6416_keypad_chip {
 	int irqnum;
 	u16 pinmask;
 	bool use_polling;
-	struct tca6416_button buttons[0];
+	struct tca6416_button buttons[];
 };
 
 static int tca6416_write_reg(struct tca6416_keypad_chip *chip, int reg, u16 val)