vdpa/mlx5: Make use of a specific 16 bit endianness API

Introduce a dedicated function to be used for setting 16 bit fields per
virio endianness requirements and use it to set the mtu field.

Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20200917121425.GA98139@mtl-vdi-166.wap.labs.mlnx


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>