net: mvpp2: unify the txq size define use

The txq size is defined by MVPP2_AGGR_TXQ_SIZE, which is sometime not
used directly but through variables. As it is a fixed value use the
define everywhere in the driver.

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