staging: comedi: usbdux: remove unnecessary check in usbdux_ai_cmd()

This check is not necessary. The cmd->chanlist_len will always be
less than or equal to the subdevice len_chanlist that was setup
during the attach of the device.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>