r8169: replace rx_buf_sz with a constant

rx_buf_sz is constant, so we don't have to pass it as parameter and
in general can replace it with a constant.

When working on this I noticed that also before in
rtl_set_rx_max_size() a value of 0x4000 is set, what is not in line
with the chip spec. According to the spec only bits 0..13 are used
and we set an effective value of zero therefore.
However, the driver still seems to work and due to potential side
effects I'm reluctant to make a change.

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