net: phy: ti: use generic helpers to access MMD registers

Now that generic helpers are available, use those instead of relying on
ti specific functions.

Signed-off-by: Carlo Caione <ccaione@baylibre.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>