staging: comedi: cb_pcidas64: remove 'volatile' from set_dac_range_bits() param

As indicated by checkpatch.pl, "WARNING: Use of volatile is usually
wrong: ...". The 'bits' parameter to this function does not need to
be volatile.

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