ssa_db.h: 'struct ssa_db' comments update

DB_VERSION_INVALID is used instead '0' as a
terminator in definition structures.

Aligned with 9b0ece5d3c99ddfd69f5e9b5e17c1615ecb289f0 commit.

Signed-off-by: Ilya Nelkenbaum <ilyan@mellanox.com>
Signed-off-by: Hal Rosenstock <hal@mellanox.com>