Aggregation buffer size is set separately per 18xx/12xx chip family.
For 18xx aggragation buffer is set to 13 pages to utilize all
the available tx/rx descriptors for aggregation.

[Arik - remove redundant parts from the patch]
Signed-off-by: NIgal Chernobelsky <igalc@ti.com>
Signed-off-by: NArik Nemtsov <arik@wizery.com>
Signed-off-by: NLuciano Coelho <luca@coelho.fi>