staging: comedi: usbduxfast: rename 'dux_commands' in private data

For aesthetic reasons, rename this variable to 'duxbuf' and also
rename the define used for its size.

This helps fix some of the > 80 char lines reported by checkpatch.pl.
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>