i40e: Fix VF set max MTU size

Max MTU sent to VF is set to 0 during memory allocation. It cause
that max MTU on VF is changed to IAVF_MAX_RXBUFFER and does not
depend on data from HW.

Set max_mtu field in virtchnl_vf_resource struct to inform
VF in GET_VF_RESOURCES msg what size should be max frame.

Fixes: dab86afd ("i40e/i40evf: Change the way we limit the maximum frame size for Rx")
Signed-off-by: Michal Jaron <michalx.jaron@intel.com>
Signed-off-by: Mateusz Palczewski <mateusz.palczewski@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>