net: phy: mscc: macsec: constify vsc8584_macsec_ops

The only usage of vsc8584_macsec_ops is to assign its address to the
macsec_ops field in the phydev struct, which is a const pointer. Make it
const to allow the compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>