net: phy: mscc: macsec: do not copy encryption keys

Following 1b16b3fd ("net: phy: mscc: macsec: clear encryption keys when freeing a flow"),
go one step further and instead of calling memzero_explicit on the key
when freeing a flow, simply not copy the key in the first place as it's
only used when a new flow is set up.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>