staging: dgnc: remove redundant NULL check for brd
the "brd" value cannot be NULL in dgnc_finalize_board_init().
Because "brd" as a parameter of this function was already
checked for NULL.
the dgnc_finalize_board_init() as a static function was called
only from dgnc_found_board() function and brd->magic value
was assigned once in dgnc_found_board(). So it doesn't need to
check for DGNC_BOARD_MAGIC value.