staging: comedi: dt282x: introduce dt282x_alloc_dma()

The IRA and DMA are optional with this driver but both are required to
support async commands. Introduce a helper function to request the IRQ
and DMA channel and allocate the buffers. Don't fail the driver attach
if the user passed an invalid IRQ or DMA channel or they cannot be
requested.

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>