r8169: replace macro TX_FRAGS_READY_FOR with a function

Replace macro TX_FRAGS_READY_FOR with function rtl_tx_slots_avail
to make code cleaner and type-safe.

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