mlx5: fix build with GCC < 4.6

mlx5: fix build with GCC < 4.6

Seen with GCC < 4.6:

 error: unknown field ‘tcp_udp’ specified in initializer
 error: extra brace group at end of initializer

Static initialization of anonymous structs/unions is a C11 feature
properly supported only since GCC 4.6.

Work around compilation errors with older versions by expanding
struct ibv_exp_flow_spec into struct hash_rxq_init.

Signed-off-by: Yaacov Hazan <yaacovh@mellanox.com>
Signed-off-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>