r8152: fix the force speed doesn't work for RTL8156

It needs to set mdio force mode. Otherwise, link off always occurs when
setting force speed.

Fixes: 195aae32 ("r8152: support new chips")
Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>