phy: lynx-28g: lock PHY while performing CDR lock workaround

lynx_28g_cdr_lock_check() runs once per second in a workqueue to reset
the lane receiver if the CDR has not locked onto bit transitions in the
RX stream. But the PHY consumer may do stuff with the PHY simultaneously,
and that isn't okay. Block concurrent generic PHY calls by holding the
PHY mutex from this workqueue.

Fixes: 8f73b37c ("phy: add support for the Layerscape SerDes 28G")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>