net: phy: mscc: macsec: clear encryption keys when freeing a flow

Commit aaab73f8 ("macsec: clear encryption keys from the stack after
setting up offload") made sure to clean encryption keys from the stack
after setting up offloading, but the MSCC PHY driver made a copy, kept
it in the flow data and did not clear it when freeing a flow. Fix this.

Fixes: 28c5107a ("net: phy: mscc: macsec support")
Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>