staging: comedi: usbdux: move and rename the bulk transfer commands

For aesthetic reasons, move the defines used for the bulk transfer
commands out of the middle of the code and rename them so they are
a bit more readable.

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>