plugin/ssa_comparison.c: Fix SMDB subnet options table generation

Currently in case of "smdb_deltas 1" and no changes occured
since last SUBNET UP event, subnet options table dataset
set_count is updated to 1, although it contains invalid
data.

In order to avoid such cases subnet options table
dataset is now updated in 2 cases:
 - There was change between current and last versions of data
 - smdb_deltas == 0

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