usbatm: create sysfs link "device" from atm class device to usb interface

There is currently no path from the ATM device in /sys to the USB device's
interface that the driver is using; this patch creates a "device" symlink.  It
is then possible to get to the cxacru ADSL statistics
(http://lkml.org/lkml/2007/2/23/328

):

/sys/class/atm/cxacru0/device $ ls *_rate *_margin *_attenuation
downstream_attenuation  downstream_snr_margin  upstream_rate
downstream_rate         upstream_attenuation   upstream_snr_margin

If this link is not appropriate I'd have to create device files in
/sys/class/atm/cxacru0 instead - which seems less appropriate since the ADSL
statistics are for the USB device not ATM (which is running over the ADSL).

[akpm@linux-foundation.org: cleanups]
Signed-off-by: Simon Arlott <simon@fire.lp0.eu>
Cc: Duncan Sands <duncan.sands@math.u-psud.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>