staging: comedi: adv_pci1710: remove unnecessary 'valid'

The 'valid' variable in the private data is only used in the
detach of the board to determine if the pci1710_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>