tsnep: Prepare RX buffer for XDP support

Always reserve XDP_PACKET_HEADROOM in front of RX buffer. Similar DMA
direction is always set to DMA_BIDIRECTIONAL. This eliminates the need
for RX queue reconfiguration during BPF program setup. The RX queue is
always prepared for XDP.

No negative impact of DMA_BIDIRECTIONAL was measured.

Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>