net: stmmac: Rework TX Coalesce logic

Coalesce logic currently increments the number of packets and sets the
IC bit when the coalesced packets have passed a given limit. This does
not reflect very well what coalesce was meant for as we can have a large
number of packets that are coalesced and then a single one, sent later
on that has the IC bit.

Rework the logic so that it coalesces only upon a limit of packets and
sets the IC bit for large number of packets.

Signed-off-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>