plugin/core.c: Add warning in case of incremental changes configured

Currently incremental changes are not supported in
SSA framework (only partially in core layer), therefore
in case of smdb_deltas other than 0, a warning is logged
and the option falls back to 0.

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