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

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

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>