phy: lynx-28g: serialize concurrent phy_set_mode_ext() calls to shared registers

In the Linux kernel, the following vulnerability has been resolved:

phy: lynx-28g: serialize concurrent phy_set_mode_ext() calls to shared registers

The protocol converter configuration registers PCC8, PCCC, PCCD
(implemented by the driver), as well as others, control protocol
converters from multiple lanes (each represented as a different
struct phy). So, if there are simultaneous calls to phy_set_mode_ext()
to lanes sharing the same PCC register (either for the "old" or for the
"new" protocol), corruption of the values programmed to hardware is
possible, because lynx_28g_rmw() has no locking.

Add a spinlock in the struct lynx_28g_priv shared by all lanes, and take
the global spinlock from the phy_ops :: set_mode() implementation. There
are no other callers which modify PCC registers.