staging: comedi: addi_apci_3120: tidy up devpriv->mode in apci3120_ai_cmd()

There is no reason for the separate updates of the mode register in this
function. Refactor the code so that the mode register is only updated at
the end of the function after all the necessary bits have been set.

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>