net: phy: Fix ethernet-phy-id <dot> in the code

Use dot instead of comma. The fix doesn't affect anything but it is good to
be aligned with used pattern. The first is used only for string size
calculation and the second change is in the comment.

Fixes: db681d49 ("net: phy: Add new read ethernet phy id function")
Signed-off-by: Michal Simek <michal.simek@amd.com>