staging: comedi dt2814: Removed unused variables

Removed unused variables.

Acked-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Fatih Yildirim <yildirim.fatih@gmail.com>
Link: https://lore.kernel.org/r/20210221202855.12442-1-yildirim.fatih@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>