net/mlx5: Properly check applicability of devlink eswitch commands

Currently we don't check that the link type is Eth and hence crash
on IB ports when attempting to deref esw->xxx, fix that.

To avoid repeating this check over and over, put the existing
checks and the one on link type in a single helper.

Fixes: 7768d197

 ('net/mlx5: E-Switch, Add control for encapsulation')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Reported-by: Mohamad Badarnah <mohamadb@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>