NET: Fix system hanging if NET device is not installed

If we try to boot from NET device, NetInitLoop in net.c will be invoked.
If NET device is not installed, eth_get_dev() function will return
eth_current value, which is NULL.
When NetInitLoop is called, "eth_get_dev->enetaddr" will access
restricted memory area and therefore cause hanging.
This issue is found on Tegra30 Cardhu platform after adding
CONFIG_CMD_NET and CONFIG_CMD_DHCP in config header file.

Signed-off-by: Jim Lin <jilin@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>