HC2VPP-354: use hw_interface_set_mtu for Ethernet interfaces

HC2VPP-354: use hw_interface_set_mtu for Ethernet interfaces

The API was renamed by
https://gerrit.fd.io/r/#/c/12930/

The VPP change spearates setting of hardware interface
and software interface MTU.

More details:
https://git.fd.io/vpp/tree/src/vnet/MTU.md

HC supports only hardware interface MTU configuration
for Ethernet interfaces.

MTU for software interfaces (per protocol MTU) is not
supported (HC2VPP-355).

Change-Id: I7eb1cb035a7b0f428a7bc7a9bb1c73819b52f0fa
Signed-off-by: Marek Gradzki <[email protected]>