staging: comedi: ni_labpc: don't clear cmd3 bits explicitly in labpc_ai_cmd()

`labpc_ai_cmd()` calls `labpc_cancel()` which already sets
`devpriv->cmd3` to 0.  Remove the lines from `labpc_ai_cmd()` that clear
specific bits in `devpriv->cmd3` explicitly as they have no effect.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>