tg3: Enable PHY reset in MTU change path for 5720

A customer noticed RX path hang when MTU is changed on the fly while
running heavy traffic with NCSI enabled for 5717 and 5719. Since 5720
belongs to same ASIC family, we observed same issue and same fix
could solve this problem for 5720.

Signed-off-by: Siva Reddy Kallam <siva.kallam@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>