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

For aesthetic reasons, rename this variable to 'inbuf' to match the
define used for its size (SIZEINBUF).

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>