wlcore/wl18xx/wl12xx: aggregation buffer size set

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: Igal Chernobelsky <igalc@ti.com>
Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luciano Coelho <luca@coelho.fi>