net: dsa: mv88e6xxx: silently skip PVT ops

Since mv88e6xxx_pvt_map is a static helper, no need to return
-EOPNOTSUPP if the chip has no PVT, simply silently skip the operation.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>