r8169: allow true forced mode setting

Due to mostly historic reasons, including a lack of reliability
of the link handling (especially with the older 8169), the
current r8169 driver emulates forced mode setting by limiting
the advertised modes.

With this change the driver allows real 10/100 forced mode
settings on the 8169 and 8101/8102.

Original idea by Vincent Steenhoute. The RTL_GIGA_MAC_VER_03
tweak was extracted from Realtek's r8169 v6.010.00 driver.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Tested-by: Jean Delvare <jdelvare@suse.de>
Cc: Edward Hsu <edward_hsu@realtek.com.tw>
Signed-off-by: David S. Miller <davem@davemloft.net>