USB: fix read vs. disconnect race in cytherm driver

the disconnect method of this driver set intfdata to NULL before
removing attribute files. The attributes' read methods will happily
follow the NULL pointer. Here's the correct ordering.

Signed-off-by : Oliver Neukum <oneukum@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>