staging: comedi: usbduxfast: remove extra dev_err() messages

A dev_err() message is output if send_dux_commands() fails. Remove
the extra dev_err() messages output by the callers.

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>