eth: mlx5: allow disabling queues when RSS contexts exist

Since commit 24ac7e54 ("ethtool: use the rss context XArray
in ring deactivation safety-check") core will prevent queues from
being disabled while being used by additional RSS contexts.
The safety check is no longer necessary, and core will do a more
accurate job of only rejecting changes which can actually break
things.

Reviewed-by: Gal Pressman <gal@nvidia.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>