staging: comedi: introduce 'comedi_board' helper function

This helper function is used to fetch the comedi_device board_ptr
which is used during the attach to pass board specific information
to the comedi drivers.

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