RDMA/uverbs: Fix the check for port number

RDMA/uverbs: Fix the check for port number

commit 5a7a88f1b488e4ee49eb3d5b82612d4d9ffdf2c3 upstream.

The port number is only valid if IB_QP_PORT is set in the mask.
So only check port number if it is valid to prevent modify_qp from
failing due to an invalid port number.

Fixes: 5ecce4c9b17b("Check port number supplied by user verbs cmds")
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Mustafa Ismail <mustafa.ismail@intel.com>
Tested-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>
[bwh: Backported to 3.2: command structure is cmd not cmd->base]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>