tg3: Preserve PCIe MPS setting for new devs

Most older tg3 devices only supported a PCIe maximum payload size of
128 bytes.  More recent devices bump this limit up to 256 bytes
though.  This patch modifies the code so that the MPS limit is only
enforced on those devices that only allow the 128 byte setting.

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