net/mlx4_core: Fix sparse warning

This patch force conversion to u32 to fix the following sparse warning:
drivers/net/ethernet/mellanox/mlx4/fw.c:1822:53: warning: restricted __be32
degrades to integer

Casting to u32 is safe here, because token will be returned as is
from the hardware without any modification.

Signed-off-by: Eyal Perry <eyalpe@mellanox.com>
Signed-off-by: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>