r8169: Fix warning in rtl8169_start_xmit().

Reported by Stephen Rothwell:

drivers/net/r8169.c: In function 'rtl8169_start_xmit':
drivers/net/r8169.c:3421: warning: label 'out' defined but not used

Introduced by commit 61357325 ("netdev:
convert bulk of drivers to netdev_tx_t").

Signed-off-by: David S. Miller <davem@davemloft.net>