staging: comedi: adl_pci9111: remove unnecessary 'is_valid'

The 'is_valid' variable in the private data is only used in the
detach of the board to determine if the pci9111_reset() function
can be called. That function only requires a valid dev->iobase
to work. Use that for the check instead and remove the unneeded
variable from the private data.

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>