staging: comedi: addi_apci_3120: don't allocate 2nd DMA buffer on failure

`apci3120_auto_attach()` 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>