staging: comedi: addi_apci_16xx: remove need for comedi_board()

Currently this driver uses the total number of channels, stored
in the boardinfo, in a number of the subdevice insn_* functions.
This information is already available in the subdevice as 's->n_chan'.
Use that instead of needing to get the board pointer first.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>