staging: comedi: adl_pci9118: don't allocate 2nd DMA buffer on failure

staging: comedi: adl_pci9118: don't allocate 2nd DMA buffer on failure

`pci9118_alloc_dma()` tries to allocate two DMA buffers but may allocate
a single buffer or none at all.  If it fails to allocate the first
buffer, it still tries to allocate the second buffer, even though it
won't be used.  Change it to not bother trying to allocate the second
buffer if the first one fails.

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