net/smc: fix direct access to ib_gid_addr->ndev in smc_ib_determine_gid()

Sparse complaints 3 times about:
net/smc/smc_ib.c:203:52: warning: incorrect type in argument 1 (different address spaces)
net/smc/smc_ib.c:203:52:    expected struct net_device const *dev
net/smc/smc_ib.c:203:52:    got struct net_device [noderef] __rcu *const ndev

Fix that by using the existing and validated ndev variable instead of
accessing attr->ndev directly.

Fixes: 5102eca9

 ("net/smc: Use rdma_read_gid_l2_fields to L2 fields")
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>