r8169: factor out rtl8169_tx_map

Factor out mapping the tx skb to a new function rtl8169_tx_map(). This
allows to remove redundancies, and rtl8169_get_txd_opts1() has only
one user left, so it can be inlined.
As a result rtl8169_xmit_frags() is significantly simplified, and in
rtl8169_start_xmit() the code is simplified and better readable.
No functional change intended.

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