net: mvpp2: allocate zeroed tx descriptors

Reserved and unused fields in the Tx descriptors should be 0. The PPv2
driver doesn't clear them at run-time (for performance reasons) but
these descriptors aren't zeroed when allocated, which can lead to
unpredictable behaviors. This patch fixes this by using
dma_zalloc_coherent instead of dma_alloc_coherent.

Fixes: 3f518509 ("ethernet: Add new driver for Marvell Armada 375 network unit")
Signed-off-by: Yan Markman <ymarkman@marvell.com>
[Antoine: commit message]
Signed-off-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>