staging: comedi: adl_pci9111: the irq is only needed for async command support

An irq is only needed for async command support, modify the attach of
the subdevices so that the command support is only hooked up if the irq
request was successful. Remove the then unnecessary sanity check in
pci9111_ai_do_cmd().

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>