nfp: correct DMA direction in XDP DMA sync

dma_sync_single_for_*() takes the direction in which the buffer
was mapped, not the direction of the sync.  We should sync XDP
buffers bidirectionally.

Fixes: ecd63a02

 ("nfp: add XDP support in the driver")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>