staging: comedi: usbduxsigma: rename {send,receive}_dux_commands()

Rename these functions so they have namespace associated with the
driver.

Also, rename the #defines used for the bulk transfer commands and
move them up with the other defines.

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>