staging: comedi: addi_apci_3501: fix digital output 'insn_bits' function

staging: comedi: addi_apci_3501: fix digital output 'insn_bits' function

This driver does not follow the comedi API. The digital output 'insn_bits'
function is passed a mask value in data[0] indicating which output bits in
data[1] are changing. The function is then supposed to update the outputs
accordingly and then return the current state of the outputs in data[1].

Currently this driver uses the 'insn_write' function to update either a
single output channel or all the channels and the 'insn_bits' function to
read the state of either a single channel or all the channels.

Fix the 'insn_bits' function so it works like the comedi core expects. The
core can then use the function to emulate the 'insn_read' and 'insn_write'
functions 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>