net: dsa: mv88e6xxx: Add missing static to stub functions

Stub functions in header files need to be static, or we can have
multiple definitions errors.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 6335e9f2

 ("net: dsa: mv88e6xxx: mv88e6390X SERDES support")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>