net/mlx5e: Fix select queue callback

The default fallback function used by mlx5e select queue can return
any TX queues in range [0..dev->num_real_tx_queues).

The current implementation assumes that the fallback function returns
a number in the range [0.. number of channels).  Actually
dev->num_real_tx_queues = (number of channels) * dev->num_tc;
which is more than the expected range if num_tc is configured and could
lead to crashes.

To fix this we test if num_tc is not configured we can safely return the
fallback suggestion, if not we will reciprocal_scale the fallback
result and normalize it to the desired range.

Fixes: 08fb1dac

 ('net/mlx5e: Support DCBNL IEEE ETS')
Signed-off-by: Rana Shahout <ranas@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Reported-by: Doug Ledford <dledford@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>