net/fungible: Fix local_memory_node error

Stephen Rothwell reported the following failure on powerpc:

ERROR: modpost: ".local_memory_node"
[drivers/net/ethernet/fungible/funeth/funeth.ko] undefined!

AFAICS this is because local_memory_node() is a non-inline non-exported
function when CONFIG_HAVE_MEMORYLESS_NODES=y. It is also the wrong API
to get a CPU's memory node. Use cpu_to_mem() in the two spots it's used.

Fixes: ee6373dd ("net/funeth: probing and netdev ops")
Fixes: db37bc17 ("net/funeth: add the data path")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Dimitris Michailidis <dmichail@fungible.com>
Signed-off-by: David S. Miller <davem@davemloft.net>