staging: comedi: ni_at_a2150: fix an always true condition test

As pointed out by the kbuild test robot, and Ian Abbott, the dma >= 0 test
is always true because dma is unsigned. This generates a warn message:

drivers/staging/comedi/drivers/ni_at_a2150.c:715 a2150_attach()
  warn: always true condition '(dma >= 0) => (0-u32max >= 0)'

Fix the test as suggested by Ian Abbott.

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