tg3: Fix NETIF_F_LOOPBACK error

Mahesh Bandewar noticed that the features cleanup in commit
0da0606f, entitled
"tg3: Consolidate all netdev feature assignments", mistakenly sets
NETIF_F_LOOPBACK by default.  This patch corrects the error.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>