dsa: mv88e6xxx: hide unused functions

dsa: mv88e6xxx: hide unused functions

When CONFIG_NET_DSA_HWMON is disabled, we get warnings about two unused
functions whose only callers are all inside of an #ifdef:

drivers/net/dsa/mv88e6xxx.c:3257:12: 'mv88e6xxx_mdio_page_write' defined but not used [-Werror=unused-function]
drivers/net/dsa/mv88e6xxx.c:3244:12: 'mv88e6xxx_mdio_page_read' defined but not used [-Werror=unused-function]

This adds another ifdef around the function definitions. The warnings
appeared after the functions were marked 'static', but the problem
was already there before that.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 57d3231057e9 ("net: dsa: mv88e6xxx: fix style issues")
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>