ax88172a: fix information leak on short answers

If a malicious device gives a short MAC it can elicit up to
5 bytes of leaked memory out of the driver. We need to check for
ETH_ALEN instead.

Reported-by:  <syzbot+a8d4acdad35e6bbca308@syzkaller.appspotmail.com>
Signed-off-by: Oliver Neukum <oneukum@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>