RDMA/bnxt_re: Use rdma_read_gid_hw_context to retrieve HW gid index

bnxt_re HW maintains a GID table with only a single entry for the two
duplicate GID entries (v1 and v2). Driver needs to map stack gid index to
the HW table gid index.  Use the new API rdma_read_gid_hw_context () to
retrieve the HW GID context to get the HW table index.

Link: https://lore.kernel.org/r/1582107594-5180-3-git-send-email-selvin.xavier@broadcom.com
Signed-off-by: Selvin Xavier <selvin.xavier@broadcom.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>