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

This member of the private data is always the cmd->chanlist_len. Use that
instead and remove the 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>