net: mscc: ocelot: use DIV_ROUND_UP helper in ocelot_port_inject_frame

This looks a bit nicer than the open-coded "(x + 3) % 4" idiom.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>