ravb: fix software timestamping

In ravb_start_xmit dont call skb_tx_timestamp only when hardware
timestamping is requested: in the latter case software timestamps are
suppressed and thus the call of skb_tx_timestamp does not have any effect.

Instead call skb_tx_timestamp unconditionally in ravb_start_xmit, since
the function checks itself if software timestamping is required or should
be skipped due to hardware timestamping.

Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Acked-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>