staging: comedi: pcl816: clarify dma channel request in pcl816_attach()

All the board types can do DMA using DMA channels 3 or 1. Remove the 'DMAbits',
which is a mask of the valid channels, from the boardinfo.

Refactor pcl816_attach() to remove the need for the goto.

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>