dpaa2-eth: make dpaa2_eth_set_dist_key static

The dpaa2_eth_set_dist_key function is only used in a single file.
Make it static.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>