staging: comedi: ni_at_2150: tidy up irq/dma request

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

Tidy up the code that does the irq and dma requests so that the
driver will still attach if they are not avaliable. The attach
will still fail, with -ENOMEM, if the dma buffer cannot be allocated.

Remove the noise about the irq and dma during the attach.

Only hook up the async commands support if the irq and dma are
available. Remove the then unnecessary sanity check in a2150_ai_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>