staging: comedi: usbduxsigma: tidy up send_dux_commands()

Rename the variable used for the private data pointer.

Remove dev_err() when usb_blk_msg() fails, it's just added noise.

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>