staging: comedi: ni_65xx: remove 'output_bits' from private data

Remove the need for the 'output_bits' in the private data by just
reading the current state of the data port when updating the output
channels in the (*insn_bits) function.

Add a local variable to handle the inverting of the hardware values
when the boardinfo indicates that the outputs are inverted.

Rename the local variable 'port_read_bits' to 'bits' so that we can
use it for updating the output channels and reading back the actual
state to return to the user.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>