staging: comedi: remove duplicate pointer assignments in attach functions

Some board pointer are assigned twice via comedi_board() in the comedi low
level driver attach functions. Remove the duplicate assignment from the
variable definition where the pointer is not used anyway until assigned later
in the function when dev->board_ptr, that comedi_board() relies on, is setup
correctly.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>