[TG3]: add 5780 basic jumbo frame support

Add basic jumbo frames support for 5780. This chip supports jumbo frames
on the standard receive ring without the jumbo ring. The
TG3_FLAG_JUMBO_ENABLE is changed to TG3_FLAG_JUMBO_RING_ENABLE to
indicate using the jumbo ring on 5704 and older chips. A new
TG3_FLG2_JUMBO_CAPABLE flag is added to indicate jumbo frames support
with or without the jumbo ring.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>