fs_enet: Don't call NAPI functions when NAPI is not used.

fs_enet_close() calls napi_disable() unconditionally. This patch skips the
call when use_napi isn't set.

Signed-off-by: Laurent Pinchart <laurentp@cse-semaphore.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>