staging: comedi: cb_pcidas64: remove board(dev) inline

The `board(dev)` inline function returns `dev->board_ptr` cast from
`const void *` to `struct pcidas64_board *`.  It really ought to return
a `const struct pcidas64_board *`.  Rather than fix the function, just
remove it and replace the calls with a local variable `thisboard` in the
functions that call it.  `thisboard` is set to the result of the common
inline function `comedi_board(dev)` defined in "comedidev.h".

Fix a little resulting fall-out from the inline function
`ai_dma_ring_count(board)` whose parameter should have been a const
pointer.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>