[TG3]: Increase the PCI MRRS

Previous devices hardcoded the PCI Maximum Read Request Size to 4K.  To
better comply with the PCI spec, the hardware now defaults the MRRS to
512 bytes.  This will yield poor driver performance if left untouched.
This patch increases the MRRS to 4K on driver initialization.

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