Staging: comedi: fix read past end of array in cb_pcidda_attach()

There are only 6 elements in the cb_pcidda_boards[] array so the
original code read past the end.  After this change nothing uses N_BOARDS
so I removed the definition.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>