staging: comedi: s626: factor out the dma buffer allocation

To make the attach a bit cleaner, factor the dma buffer allocation
out of attach_pci() into a new function.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>