net: mvpp2: set SKBTX_IN_PROGRESS

Richard Cochran points out that SKBTX_IN_PROGRESS should be set when
the skbuff is queued for timestamping.  Add this.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>