net: mvpp2: remove support for buffer header

The "buffer header" functionality is a functionality used by the
hardware to split an incoming packets over multiple BM buffers if they
are not large enough. However, the mvpp2 driver guarantees that a pool
of BM buffers has buffers with a size large enough to store MTU-sized
packets. Therefore, this functionality is completely unused, and the
code can be removed, and we should never get a descriptor with bit
MVPP2_RXD_BUF_HDR set.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>