staging: comedi: addi_apci_3501: 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 driver uses the data[0] value passed to the function
to determine if a single input channel status is requested or the status of
all the input channels.

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>