staging: comedi: dt2811: remove disabled interrupt support code

The interrupt support in this driver is incomplete due to the missing
comedi functions to support async commands.

Since the interrupt code is already disable just remove it.

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>