net: mvpp2: fix TSO headers allocation and management

TSO headers are managed with txq index and therefore should be aligned
with the txq size, not with the aggregated txq size.

Fixes: 186cd4d4

 ("net: mvpp2: software tso support")
Reported-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Yan Markman <ymarkman@marvell.com>
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>