staging: line6: do not return 0 from probe if no initialization done

There is a strange "return 0" in line6_probe() before any initialization of
the module is done. It can lead to NULL pointer dereference in other functions.
The patch proposes to return -ENODEV in this case.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>