staging: comedi: addi_apci_3120: fix cmd->convert_arg vaildation

Regardless of the cmd->scan_begin_src, the minimum cmd->convert_arg
value is 10000 (10us). Fix the validation in apci3120_ai_cmdtest().

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>