staging: comedi: icp_multi: remove 'valid' member from private data

This member is set at the end of the driver attach and is only used to
verify that icp_multi_reset() can be called in the (*detach). The only
requirement for icp_multi_reset() to work is that the dev->mmio is valid.

Remove the 'valid' member from the private data and use 'dev->mmio' to
see if the reset can be done.

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