net/mlx5: E-Switch, return error if encap isn't supported

On regular ConnectX HCAs getting encap mode isn't supported when the
E-Switch is in NONE mode. Current code would return no error code when
trying to get encap mode in such case which is wrong.

Fix by returning error value to indicate failure to caller in such case.

Fixes: 8e0aa4bc

 ("net/mlx5: E-switch, Protect eswitch mode changes")
Signed-off-by: Raed Salem <raeds@nvidia.com>
Reviewed-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Maor Dickman <maord@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>