staging: comedi: dt9812: convert digital in subdevice to (*insn_bits)

Currently the (*insn_read) function for the digital input subdevice
returns the state for a single channel. It's more efficent to use
the (*insn_bits) function and return the state for all the channels.

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

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>