staging: comedi: usbduxfast: tidy up send_dux_commands()

Rename this function so it has namespace associated with the driver.

For aesthetic reasons, rename the local variable 'tmp' to 'ret' as
this is more common for checking errno values.

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>