USB: cxacru: create sysfs attributes in atm_start instead of bind

Since usbatm doesn't set the usb_interface driver data until after calling
bind and heavy_init, it would be NULL when the sysfs attributes are read.
Reading the MAC address from atm_dev before atm_dev exists would have been
be possible too.

Calling create_device_file in atm_start will avoid this problem, and the
data is useless until the first status poll runs.  However, it must be
ready before a status poll does a printk on line status change otherwise
userspace could react before the files exist.

For completeness I've moved remove_device_file to atm_stop so it's not
called in unbind when it's not needed.  There's no point starting ADSL if
atm_start could still fail either.

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>