media: siano: Use flex arrays for sms_firmware

Replace old style single array member, with flex array.

The struct is allocated, but it seems like there was an over allocation
error:

fw_buf = kmalloc(ALIGN(fw->size + sizeof(struct sms_firmware),
		SMS_ALLOC_ALIGNMENT), GFP_KERNEL | coredev->gfp_buf_flags);

This change fixes this cocci warning:
drivers/media/common/siano/smscoreapi.h:669:6-13: WARNING use flexible-array member instead (https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays)
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>

drivers/media/common/siano/smscoreapi.h

@@ -666,7 +666,7 @@ struct sms_firmware {
 	u32			check_sum;
 	u32			length;
 	u32			start_address;
-	u8			payload[1];
+	u8			payload[];
 };
 
 /* statistics information returned as response for