staging: comedi: ni_mio_common: merge analog output (*insn_write) functions

staging: comedi: ni_mio_common: merge analog output (*insn_write) functions

Currently the PCI6xxx boards use a different (*insn_write) function
for the analog output subdevice. Most of the code is identical to the
(*insn_write) used by all the other board types.

For aesthetics, merge the two (*insn_write) functions and handle the
'is_6xxx' differences in the common code.

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>