net/mlx5e: Permit XDP with non-linear legacy RQ

Now that legacy RQ implements XDP in the non-linear mode, stop blocking
this configuration. Allow non-linear mode only for programs aware of
multi buffer.

XDP performance with linear mode RQ hasn't changed.

Baseline (MTU 1500, TX MPWQE, legacy RQ, single core):
 60-byte packets, XDP_DROP: 11.25 Mpps
 60-byte packets, XDP_TX: 9.0 Mpps
 60-byte packets, XDP_PASS: 668 kpps

Multi buffer (MTU 9000, TX MPWQE, legacy RQ, single core):
 60-byte packets, XDP_DROP: 10.1 Mpps
 60-byte packets, XDP_TX: 6.6 Mpps
 60-byte packets, XDP_PASS: 658 kpps
 8900-byte packets, XDP_DROP: 769 kpps (100% of sent packets)
 8900-byte packets, XDP_TX: 674 kpps (100% of sent packets)
 8900-byte packets, XDP_PASS: 637 kpps

Signed-off-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>