net: systemport: correctly check for RX_STATUS_OVFLOW flag

We were missing an and comparison with status to check whether
RX_STATUS_OVFLOW is asserted or not in the per-packet status word, fix
that.

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