get_log_timestamp API is being called unconditionally when passed as
an argument to dp_pkt_add_timestamp (even for empty calls) from
dp_tx_hw_enqueue_li. This adds significant load on CPU, hence avoid
get_log_timestamp call by using preprocessor directive.
Change-Id: I1d988597e28b755a767a267d1b578b2bcedc324e
CRs-Fixed: 3150795