The whole purpose of xgi_pcie_heap_check is to log information about
entries on the used_list. If XGI_DEBUG is not set, it doesn't print
anything. Therefore we can #ifdef the whole function body.
Convert open-code list iteration to use list_for_each_entry.
This function used to return 'void *', which was then cast to
'xgi_pcie_block_t *' at the only caller. I changed the return type to
'struct xgi_pcie_block_s *' and removed the explicit cast.
