staging: comedi: pcl816: remove unreachable interrupt handler code

According to the users manual, when using interrupt (non-DMA) transfers
this hardware generates an interrupt at the start of a conversion. This
requires the interrupt routine to busywait until the end-of-conversion.
It appears this was unreliable and interrupt only async command support
was removed at some time. Async command support is still available when
DMA is used.

Remove the unreachable interrupt only handler code.

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>