staging: comedi: usbduxsigma: remove unnecessary check in usbdux_ao_cmd()

The comedi core verifies that the command chanlist_len is valid for
the subdevice based on the len_chanlist that was setup during the
attach. There is no need to recheck it in the (*cmd) function.

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>