staging: comedi: usbduxfast: rename 'insnBuffer' from private data

This buffer space is kmalloc'ed and free'ed but never used. Just
remove it.

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>