net/ethernet: Fix VLAN networking on little-endian systems

net/ethernet: Fix VLAN networking on little-endian systems

VLAN configuration seems to have never worked on little-endian systems.
This is likely because VLANTAG_IDENTIFIER is not byte-swapped before
copying into the net buffer, nor is inf->vlantag. We can resolve this by
using grub_cpu_to_be16{_compile_time}() and its inverse when copying
VLAN info to/from the net buffer.

Signed-off-by: Chad Kimes <chkimes@github.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>