staging: comedi: addi-data: remove 'b_ValidDriver' from the private data

This variable is only used in the detach of the addi-data boards to
indicate that the attach of the board was successful and that the reset
function can be called. We can use the dev->iobase variable to indicate
this instead. Once this variable is set the attach always completes.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>