r8169: Do not increment tx_dropped in TX ring cleaning

rtl8169_tx_clear_range() is responsible for cleaning up the TX ring
during interface shutdown, incrementing tx_dropped for every SKB that we
left at the time in the ring is misleading.

Fixes: cac4b22f ("r8169: do not account fragments as packets")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>