net: phy: marvell: don't use empty switch default case

This causes error reported by kernel test robot.

Signed-off-by: Marek Behún <kabel@kernel.org>
Fixes: 41d26bf4

 ("net: phy: marvell: refactor HWMON OOP style")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>