dsa: mv88e6352/mv88e6xxx: Move temperature sensor code to mv88e6xxx.c

Move the temperature sensing code for mv88e6352 and mv88e6320 families
into mv88e6xxx.c to simplify adding support for additional chips.

With this change, mv88e6xxx_6320_family() no longer needs to be
a global function and is made static.

Cc: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>