amd-xgbe: Perform Tx coalescing on a packet basis

The current form of Tx coalescing works on a descriptor basis instead
of on a packet basis and doesn't take into account TSO packets. Update
the Tx coalescing support to work on a packet basis, taking into
account the number of packets associated with a TSO transmit. Also,
only activate the Tx timer if a timer value is set.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>