net: dsa: mv88e6xxx: Hold mutex while doing stats operations

Until now, there has been no need to hold the reg mutex while getting
the count of statistics, or the strings, because the hardware was not
accessed. When adding support for SERDES statistics, it is necessary
to access the hardware, to determine if a port is using the SERDES
interface. So add mutex lock/unlocks.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>