r8169: Kill SafeMtu macro

After d58d46b5 (r8169: jumbo fixes.) max frame len is stored in
rtl_chip_infos[].jumbo_max for each chip and SafeMtu should be gone.

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>