sfc: support for devlink port requires MAE access

On systems without MAE permission efx->mae is not initialised,
and trying to lookup an mport results in a NULL pointer
dereference.

Fixes: 25414b2a

 ("sfc: add devlink port support for ef100")
Signed-off-by: Martin Habets <habetsm.xilinx@gmail.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>