macsec: fix detection of RXSCs when toggling offloading

macsec_is_configured incorrectly uses secy->n_rx_sc to check if some
RXSCs exist. secy->n_rx_sc only counts the number of active RXSCs, but
there can also be inactive SCs as well, which may be stored in the
driver (in case we're disabling offloading), or would have to be
pushed to the device (in case we're trying to enable offloading).

As long as RXSCs active on creation and never turned off, the issue is
not visible.

Fixes: dcb780fb ("net: macsec: add nla support for changing the offloading selection")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Antoine Tenart <atenart@kernel.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>