Staging: line6: NULL dereference in dev_err()

"line6" hasn't been set at this point and we should be using
&interface->dev instead.

Gcc would have complained about this if it weren't for the fact that we
initialized line6 to NULL.  I removed the initialization.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>