staging: comedi: das1800: tidy up irq request

This driver only needs an irq in order to support async commands.
If the irq is not available the driver will still function for
single analog input reads.

Tidy up the code that does the irq requests so that the driver
will still attach if it is not avaliable.

Remove the noise about the irq during the attach.

Only hook up the async commands support if the irq is available.
Remove the then unnecessary sanity check in das1800_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>