staging: comedi: addi_apci_3120: simplify setting of devpriv->us_UseDma

`apci3120_auto_attach()` first sets `devpriv->us_UseDma` to 1, then sets
it back to 0 if it fails to allocate the DMA buffer.  Since `*devpriv`
is initially zeroed out by `comedi_alloc_devpriv()`, change it to only
set `devpriv->us_UseDma` to 1 if the allocation succeeds.  Also, don't
bother explicitly initializing `devpriv->b_DmaDoubleBuffer` to 0 as it
is already zeroed out.

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>