net: phy: use ethtool string helpers

These are the preferred way to copy ethtool strings.

Avoids incrementing pointers all over the place.

Signed-off-by: Rosen Penev <rosenp@gmail.com>
Link: https://patch.msgid.link/20241029234641.11448-1-rosenp@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>