net: phy: adin: const-ify static data

Some bits of static data should have been made const from the start.
This change adds the const qualifier where appropriate.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>