net: ethernet: ti: cpsw: fix sw timestamping for non PTP packets

The cpts can timestmap only ptp packets at this moment, so driver
cannot mark every packet as though it's going to be timestamped,
only because h/w timestamping for given skb is enabled with
SKBTX_HW_TSTAMP. It doesn't allow to use sw timestamping, as result
outgoing packet is not timestamped at all if it's not PTP and h/w
timestamping is enabled. So, fix it by setting SKBTX_IN_PROGRESS
only for PTP packets.

Signed-off-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>