staging: comedi: pcl818: remove 'irq_free' from private data

This member of the private data is not needed. We can check if
dev->irq is none zero instead. Remove it.

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>