net/mlx4_en: Remove BUG_ON assert when checking if ring is full

In mlx4_en_is_ring_empty we check if ring surpassed its size.
Since the prod and cons indicators are u32, there might be a state where
prod wrapped around and cons, making this assert false, although no
actual bug exists (other code segment can cope with this state).

Signed-off-by: Ido Shamay <idos@mellanox.com>
Signed-off-by: Eugenia Emantayev <eugenia@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>