staging: comedi: pcl818: rename 'irq_blocked' in private data

This member in the private data is a flag that indicates that an analog
input async command is currently running. Rename it to make this clear.

The private data is kzalloc'ed in the attach so remove the unnecessary
clearing of this flag.

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>