tg3: Don't check undefined error bits in RXBD

[ Upstream commit d7b95315cc7f441418845a165ee56df723941487 ]

Redefine the RXD_ERR_MASK to include only relevant error bits. This fixes
a customer reported issue of randomly dropping packets on the 5719.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>