virtio-net: make array guest_offloads static

The array guest_offloads is local to the source and does not need to
be in global scope, so make it static. Also tweak formatting.

Cleans up sparse warnings:
symbol 'guest_offloads' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>