ethtool: add tx aggregation parameters

Add the following ethtool tx aggregation parameters:

ETHTOOL_A_COALESCE_TX_AGGR_MAX_BYTES
Maximum size in bytes of a tx aggregated block of frames.

ETHTOOL_A_COALESCE_TX_AGGR_MAX_FRAMES
Maximum number of frames that can be aggregated into a block.

ETHTOOL_A_COALESCE_TX_AGGR_TIME_USECS
Time in usecs after the first packet arrival in an aggregated
block for the block to be sent.

Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>