staging: comedi: usbduxsigma: tidy up usbdux_ai_insn_read()

Rename the function so it has namespace associated with the driver.
Rename the local variable used for the private data pointer.

Return -EBUSY if an asynchronous command is running and the read cannot
be completed.

Propagate the errno if the receive_dux_command() fails.

Tidy up the function to make it more concise.

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>