tg3: Create tg3_rings_reset()

This patch moves most of the chip ring setup logic into a separate
function.  This will make it easier to verify the multi ring setup
changes.

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