davinci: omapl138_lcdk: keep booting even when MAC address is invalid

If the MAC address specified on the EEPROM is invalid (multicast or
zero address), then u-boot fails to boot. Having a bad MAC address
in the EEPROM should not prevent the system from booting.

This commit changes the error path to just print an error messages
in case of bad MAC address.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Tom Rini <trini@konsulko.com>