tg3: Fix poor tx performance on 57766 after MTU change

GRC reset causes the read DMA engine to go into a mode that breaks up
requests into 256 bytes.  A PHY reset is required to bring it back to
the normal mode.

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