r8169: improve RTL8411b phy-down fixup

Mirsad proposed a patch to reduce the number of spinlock lock/unlock
operations and the function code size. This can be further improved
because the function sets a consecutive register block.

Suggested-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Mirsad Todorovac <mirsad.todorovac@alu.unizg.hr>
Signed-off-by: David S. Miller <davem@davemloft.net>