atm: expose ATM device index in sysfs

It's currently exposed only through /proc which, besides requiring
screen-scraping, doesn't allow userspace to distinguish between two
identical ATM adapters with different ATM indexes.  The ATM device index
is required when using PPPoATM on a system with multiple ATM adapters.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Reviewed-by: Eric Dumazet <eric.dumazet@gmail.com>
Tested-by: David Woodhouse <dwmw2@infradead.org>
Cc: stable@kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>