IB/core: Fix uninitialized variable use in check_qp_port_pkey_settings

Check the return value from get_pkey_and_subnet_prefix to prevent using
uninitialized variables.

Fixes: d291f1a6 ("IB/core: Enforce PKey security on QPs")
Signed-off-by: Daniel Jurgens <danielj@mellanox.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: James Morris <james.l.morris@oracle.com>