net: systemport: Pre-calculate and utilize cb->bd_addr

There is a 1:1 mapping between the software maintained control block in
priv->rx_cbs and the buffer address in priv->rx_bds, such that there is
no need to keep computing the buffer address when refiling a control
block.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>