staging: comedi: adv_pci1710: cmd->scan_begin_src can only be TRIG_FOLLOW

In Step 1 of the (*do_cmdtest), the cmd->scan_begin_src is checked to
only allow TRIG_FOLLOW. The (*do_cmd) does not need to recheck this.

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>