net: bcmgenet: device stats are unsigned long

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

Fixes: 1c1008c7 ("net: bcmgenet: add main driver file")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>