staging: comedi: addi_apci_3120: DMA requires an interrupt

An interrupt is required for DMA to work.

Factor out the DMA buffer allocation from the (*auto_attach) and only
allocate the buffers if the interrupt is available.

For aesthetics, also factor the DMA buffer free from the (*detach).

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>