net/flower: Fix pointer cast

Cast pointer to unsigned long instead of u64, to fix compilation warning
on 32 bit arch, spotted by 0day build.

Fixes: 5b33f488 ("net/flower: Introduce hardware offload support")
Signed-off-by: Amir Vadai <amir@vadai.me>
Signed-off-by: David S. Miller <davem@davemloft.net>