dpaa2-eth: increase the needed headroom to account for alignment

dpaa2-eth: increase the needed headroom to account for alignment

Increase the needed headroom to account for a 64 byte alignment
restriction which, with this patch, we make mandatory on the Tx path.
The case in which the amount of headroom needed is not available is
already handled by the driver which instead sends a S/G frame with the
first buffer only holding the SW and HW annotation areas.

Without this patch, we can empirically see data corruption happening
between Tx and Tx confirmation which sometimes leads to the SW
annotation area being overwritten.

Since this is an old IP where the hardware team cannot help to
understand the underlying behavior, we make the Tx alignment mandatory
for all frames to avoid the crash on Tx conf. Also, remove the comment
that suggested that this is just an optimization.

This patch also sets the needed_headroom net device field to the usual
value that the driver would need on the Tx path:
	- 64 bytes for the software annotation area
	- 64 bytes to account for a 64 byte aligned buffer address

Fixes: 6e2387e8f19e ("staging: fsl-dpaa2/eth: Add Freescale DPAA2 Ethernet driver")
Closes: https://lore.kernel.org/netdev/aa784d0c-85eb-4e5d-968b-c8f74fa86be6@gin.de/
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>