staging: comedi: vmk80xx: remove digital output (*insn_read)

The comedi core can use the (*insn_bits) function to emulate the
(*insn_read) function.

The digital output (*insn_bits) function properly handles the
VMK8061_MODEL to read the digital output states before returning.

Remove the unnecessary (*insn_read) function. It's also not
necessary to set the SDF_READABLE flag so remove the entire
conditional in the attach.

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>