r8152: don't enable napi before rx ready

Adjust napi_disable() and napi_enable() to avoid r8152_poll() start
working before rx ready. Otherwise, it may have race condition for
rx_agg.

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