mac: Fix the condition check for setting the MAC from the EEPROM

The issue got introduced in a cleanup by Manjunath Hadli in commit
826e9913. The eth_getenv_enetaddr_by_index
method will validate the MAC addr and if none is set in the environment
0 will be returned. Set the MAC from the eeprom if no valid address
is found in environment.

Signed-off-by: Holger Hans Peter Freyther <holger@freyther.de>