net: dsa: mv88e6xxx: factorize temperature access

Add MV88E6XXX_FLAG_TEMP and MV88E6XXX_FLAG_TEMP_LIMIT flags to describe
switch models featuring a temperature access. Use them to centralize the
access to the temperature feature.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>