staging: comedi: addi_apci_1516: fix digital input 'insn_bits' function

This driver does not follow the comedi API. The digital input 'insn_bits'
function is supposed to return the status of all the input channels in
data[1]. Currently this function returns the status in data[0].

Fix the function so it works like the comedi core expects. The core can
then use the function to emulate the 'insn_read' function for individual
channels.

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>