staging: comedi: usbduxsigma: tidy up analog output (*insn_{read, write})

Rename the functions so they have namespace associated with the driver.
Rename the local variable used for the private data pointer.

Return -EBUSY from the (*insn_write) function if an asynchronous command
is running that the write cannot be completed.

Don't save the cached value of the analog output for readback unless
the command to set it is successful.

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>