net: dsa: bcm_sf2: Fix a possible memory leak in bcm_sf2_mdio_register()

In the Linux kernel, the following vulnerability has been resolved:

net: dsa: bcm_sf2: Fix a possible memory leak in bcm_sf2_mdio_register()

bcm_sf2_mdio_register() calls of_phy_find_device() and then

phy_device_remove() in a loop to remove existing PHY devices.

of_phy_find_device() eventually calls bus_find_device(), which calls

get_device() on the returned struct device * to increment the refcount.

The current implementation does not decrement the refcount, which causes

memory leak.

This commit adds the missing phy_device_free() call to decrement the

refcount via put_device() to balance the refcount.