net: mvpp2: report the tx-usec coalescing information to ethtool

This patch adds the tx-usec value to the informations reported to
ethtool by the get_coalesce function.

Suggested-by: Yan Markman <ymarkman@marvell.com>
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>