staging: comedi: addi_common: allow driver to set the board_ptr

The addi_apci_035, addi_apci_1500, addi_apci_1564, and addi_apci_3xxx
drivers still use the addi_common code. Allow those drivers to set
the dev->board_ptr before calling addi_auto_attach().

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>