staging: comedi: dt2815: convert printk's in dt2815_attach()

These messages don't appear to be important enough to warrent KERN_INFO
and KERN_WARNING levels. Convert them dev_dbg().

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>