net: pcs: xpcs: use mdiobus_c45_addr in xpcs_{read,write}

commit 679e283e

 upstream.

Use the dedicated helper for abstracting away how the clause 45 address
is packed in reg_addr.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Yongxin Liu <yongxin.liu@windriver.com>