r8169: improve rtl8169_rx_fill

We have only one user of the error path, so we can inline it.
In addition the call to rtl8169_make_unusable_by_asic() can be removed
because rtl8169_alloc_rx_data() didn't call rtl8169_mark_to_asic() yet
for the respective index if returning NULL.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>