Fix endianness conversions in rtl8169 driver

It is unclear on what platforms this driver has been tested, since
noone up to now defines CONFIG_RTL8169 in the board configuration
header. Now it has been fixed for a big-endian mpc8241 based
linkstation platform. This patch presents the necessary endianness
conversion fixes.

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>