staging: comedi: ni_660x: fix missing break in switch statement

Add missing break statement in order to prevent the code from falling
through to the default case and return -EINVAL every time.

This bug was found thanks to the ongoing efforts to enable
-Wimplicit-fallthrough.

Fixes: aa94f288 ("staging: comedi: ni_660x: tidy up ni_660x_set_pfi_routing()")
Cc: stable@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>