IB/mlx5: Don't override existing ip_protocol

Two flow specifications can set the ip protocol field in
the flow table entry:

1) IB_FLOW_SPEC_TCP/UDP/GRE - set the ip protocol accordingly.
2) IB_FLOW_SPEC_IPV4/6 - has ip_protocol field for users
who want to receive specific L4 packets.

We need to avoid overriding of the ip_protocol with zeros,
in case that the user first put the L4 specification and
only then the L3.

Fixes: ca0d4753 ('IB/mlx5: Add support in TOS and protocol to flow steering')
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>