mlx5: Fix incorrect wc pkey_index assignment for GSI messages

Since patch series "Demux IB CM requests in the rdma_cm module" the
P_Key index is taken from the work completion rather than the message
itself.

The HCA provides us with the message P_Key. In order to provide the
P_Key index, we need to look it up. Given that this is relevant only
for GSI messages (session establishments) which is less performance critical,
micro-optimize against the GSI (is_qp1) branch.

Fixes: 4c21b5bc ("IB/cma: Add net_dev and private data checks to
RDMA CM")
Signed-off-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>