refactor db_get_auth_data return val

Adopt the error handling of the other db functions: return -ENOENT on unknown
subscriber and -EIO on SQL failure. Return 0 for no error, instead of the
number of rows modified.

Adjust the single caller: db_get_auc()
(and db_test.c).

Change-Id: I006f471962bdad95d00a3a4c41a28ebbc9740884
3 files changed