net: mvpp2x: decrease size of AGGR_TXQ and CPU_DESC_CHUNK

U-boot use single physical tx queue with size 16 descriptors.
So aggregated tx queue size should be equal to physical tx queue
and cpu descriptor chunk(number of descriptors delivered from
physical tx queue to aggregated tx queue by one chunk) shouldn't be
larger than physical tx queue.

Fix:
Set AGGR_TXQ and CPU_DESC_CHUNK to be 16 descriptors, same as
physical TXQ.

Signed-off-by: Stefan Chulski <stefanc@marvell.com>
Tested-by: iSoC Platform CI <ykjenk@marvell.com>
Reviewed-by: Nadav Haklai <nadavh@marvell.com>
Reviewed-by: Igal Liberman <igall@marvell.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Stefan Roese <sr@denx.de>