zd1201: treat firmware data as const

Signed-off-by: David Woodhouse <[e-mail]>

drivers/net/wireless/zd1201.c

···
 static int zd1201_fw_upload(struct usb_device *dev, int apfw)
 {
 	const struct firmware *fw_entry;
-	char *data;
+	const char *data;
 	unsigned long len;
 	int err;
 	unsigned char ret;