r8152: Disable PLA MCU clock speed down

commit 08997b5e

 upstream.

PLA MCU clock speed down could only be enabled when tx/rx are disabled.
Otherwise, the packet loss may occur.

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>