net: phy: xilinx: Break while loop over ethernet phy

The commit 6c993815 ("net: phy: xilinx: Be compatible with live OF
tree") change driver behavior to while loop which wasn't correct because
the driver was looping over again and again. The reason was that
ofnode_valid() is taking 0 as correct value.
Fix it by changing while loop to ofnode_for_each_subnode() which is only
loop over available nodes.

Fixes: 6c993815 ("net: phy: xilinx: Be compatible with live OF tree")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>