net: systemport: Simplify circular pointer arithmetic

Similar to c298ede2

 ("net: bcmgenet: simplify circular pointer
arithmetic") we don't need to complex arthimetic since we always have a
ring size that is a power of 2.

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