virtio: fix 32-bit build for 64-bit kernel

virtio: fix 32-bit build for 64-bit kernel

virtio_net_hdr_mem member within virtqueue structure stores a
physical address and is defined as void ptr. When 32bit pmd is used
with 64bit kernel this leads to truncation of 64bit physical address
and pkt i/o does not work.
Changed virtio_net_hdr_mem to phys_addr_t type and
removed the typecasts

Signed-off-by: Vijayakumar Muthuvel Manickam <mmvijay@gmail.com>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>