hash: fix doxygen of return values

hash: fix doxygen of return values

rte_hash_lookup_data() and rte_hash_lookup_with_hash_data()
functions return the index of the table where the key is stored
when this is found, and not 0 as the Doxygen currently states.

Also, these functions, and rte_hash_get_key_with_position()
return negative values when keys are not found (-EINVAL and -ENOENT),
where the minus sign was missing.

Bugzilla ID: 78
Fixes: 473d1bebce43 ("hash: allow to store data in hash table")
Fixes: 6dc34e0afe7a ("hash: retrieve a key given its position")
Cc: stable@dpdk.org
Reported-by: Petr Houska <t-pehous@microsoft.com>
Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>