ice: Add support for changing MTU on PR in switchdev mode

This change adds support for changing MTU on port representor in
switchdev mode, by setting the min/max MTU values on port representor
netdev. Before it was possible to change the MTU only in a limited,
default range (68-1500).

Signed-off-by: Marcin Szycik <marcin.szycik@intel.com>
Tested-by: Sandeep Penigalapati <sandeep.penigalapati@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>