dm9601: fix IFF_ALLMULTI handling

Pass-all-multicast is controlled by bit 3 in RX control, not bit 2
(pass undersized frames).

Reported-by: Joseph Chang <joseph_chang@davicom.com.tw>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: David S. Miller <davem@davemloft.net>