net: mdio-uclass: rewrite dm_mdio_post_probe using strlcpy

dm_mdio_post_probe used to be vulnerable after truncation, but has been
patched by commit 398e7512 ("net: Fix Covarity Defect 244093").
Nonetheless, we can use strlcpy like the rest of the code base now,
which yields the same result.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>