tg3: Disable unused transmit rings

This patch allows the driver to disable the additional transmit rings
available on the 5717 and 5719 devices.  This is not strictly necessary,
but is done anyways for correctness.

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