staging: comedi: addi_apci_3120: remove private data 'b_ExttrigEnable'

This member of the private data is not really needed. The devpriv->ctrl
value can be checked to determine if the external trigger is enabled.
Remove the unnecessary member.

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>