eal/linux: fix socket value for undetermined numa node

The patch sets zero as the default value of pci device numa_node
if the socket could not be determined.
It provides the same default value as FreeBSD which has no NUMA support,
and makes the return value of rte_eth_dev_socket_id() be consistent
with the API description.

Signed-off-by: Cunming Liang <cunming.liang@intel.com>