mvneta: MVNETA_SKB_HEADROOM set last 3 bits to zero

For XDP the MVNETA_SKB_HEADROOM is used as an offset for
the received data.
The MVNETA manual states that the last 3 bits assumed to be 0.

This is currently the case but lets make it explicit in the definition
to prevent future problems.

Signed-off-by: Sven Auhagen <sven.auhagen@voleatech.de>
Signed-off-by: David S. Miller <davem@davemloft.net>