IB/mlx4: Fix wrong usage of IPv4 protocol for multicast attach/detach

The MLX4_PROT_IB_IPV4 protocol should only be used with RoCEv2 and such.
Removing this wrong usage allows to run multicast applications over RoCE.

Fixes: d487ee77 ("IB/mlx4: Use IBoE (RoCE) IP based GIDs in the port GID table")
Reported-by: Carol Soto <clsoto@linux.vnet.ibm.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>