net: bcmsysport: Device stats are unsigned long

On 64bits kernels, device stats are 64bits wide, not 32bits.

Fixes: 80105bef ("net: systemport: add Broadcom SYSTEMPORT Ethernet MAC driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>