net: kernel-doc compliant documentation for net_device

Net_device is a vast and important structure, but it has no kernel-doc
compliant documentation. This patch extracts the comments from the structure
to clean it up, and let the scripts extract documentation from it. I know that
the patch is big, but it's just reordering of comments into the appropriate
form, and adding a few more, for the missing members.

Signed-off-by: Karoly Kemeny <karoly.kemeny@gmail.com>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>