staging/comedi: Remove if(); statement without effect

Smatch warns about:
staging/comedi/drivers/cb_pcidas64.c:3304 prep_ao_dma() warn: if();

So the check currently does nothing and can be removed, as indicated by
Ian.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>