Staging: comedi: pcl818: Tidy up AI command after channel dropout or similar error.

It was causing subsequent commands to fail with -EBUSY.

From: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Frank Mori Hess <fmhess@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>