staging: comedi: hwdrv_apci1500: use dev->class_dev in calls to dev_warn()

git-grep reveals that hwdrv_apci1500.c is the only file in comedi that uses
dev->hw_dev in calls to dev_{err,warn}().  The rest of the drivers pass
dev->class_dev to these macros instead.  Switch the dev_warn() calls in
this driver to use dev->class_dev as well, for consistency.
Signed-off-by: Chase Southwood <chase.southwood@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>