r8169: checks against wrong mac addresse init

Checking the signature of the eeprom and the validity of the
MAC address should be enough to filter out the bad addresses
observed so far.

Contributed by Ivan Vecera and Martin Capitanio.

Tested on 8102el, 8168b and 8169 for a start.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Edward Hsu <edward_hsu@realtek.com.tw>