net: mvpp2: use the aggr txq size define everywhere

Cosmetic patch using the MVPP2_AGGR_TXQ_SIZE everywhere instead of the
size field of aggr_txq, as the size never change and is always equal to
the MVPP2_AGGR_TXQ_SIZE define.

Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>