staging: comedi: adv_pci_dio: checkpatch.pl cleanup (fallthrough)

Fix these checkpatch.pl warnings:

WARNING: Possible switch case/default not preceeded by break or fallthrough comment

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>