staging: comedi: usbduxfast: remove unreachable code in usbduxfast_ai_cmd()

The (*do_cmdtest) validates that the 'stop_arg' is >= 1 for a 'stop_src'
of TRIG_COUNT and the 'scan_end_arg' is validated to be the 'chanlist_len'
which will always be >= 1. Remove the unreachable error code for an
'ai_sample_count < 1'.

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>