staging: comedi: ni_at_a2150: cleanup a2150_probe()

Make this function return a pointer to the boardinfo instead of an index.

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