switchdev: Fix return value of switchdev_port_fdb_dump().

This patch fixes the retun value of switchdev_port_fdb_dump() when
CONFIG_NET_SWITCHDEV is not set. This avoids getting "warning: return makes
integer from pointer without a cast [-Wint-conversion]" when building
when CONFIG_NET_SWITCHDEV is not set under several compiler versions.
This warning is due to commit d297653d
("rtnetlink: fdb dump: optimize by saving last interface markers").

Signed-off-by: Rami Rosen <rami.rosen@intel.com>
Acked-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>