staging: comedi: usbduxfast: 'cmd->chanlist_len' was already validataed

Remove the unnecessary check of 'cmd->chanlist_len > 0'. The (*do_cmdtest) already
validated this before the (*do_cmd) was called.

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>