staging: comedi: dt3000: change return type of dt3k_send_cmd to void

None of the callers check the return value. Just make it a void.

Flip the status check at the end of the function to check for an
error before outputing the dev_dbg(). Use __func__ for the function
name in the message.

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>