r8169: add checking driver's runtime pm status in rtl8169_get_ethtool_stats()

Not to call rtl8169_update_counters() to dump tally counter when driver
is in runtime suspend state.

Calling rtl8169_update_counters() in runtime suspend state will produce
warning message "rtl_counters_cond == 1 (loop: 1000, delay: 10)".

Signed-off-by: Chunhao Lin <hau@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>