virtio-net: make net_hdr_size a variable
virtio-net: make net_hdr_size a variable

Size of the net_hdr_size is different in legacy and modern devices. This
helps in conversion of the driver to 1.0.

Signed-off-by: Nikunj A Dadhania <e2f4a5db9b4735182a88051d1e7a3599f9eac9db@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <9e65be74847981e55b2564b6266c57d86d3bac05@redhat.com>
Signed-off-by: Alexey Kardashevskiy <ac1fc56530d7d037e1ea666f130daca22d72f964@ozlabs.ru>

open-power-host-os/slof,qemu/SLOF,aik/SLOF,qemu/SLOF,stefanberger/SLOF-tpm,open-power-host-os/slof,aik/SLOF,aik/SLOF,qemu/SLOF,stefanberger/SLOF-tpm,stefanberger/SLOF-tpm,open-power-host-os/slof,open-power-host-os/slof