net: eth: Check return value in various places

eth_get_dev() can return NULL which means device_probe() fails for
that ethernet device. Add return value check in various places or
U-Boot will crash due to NULL pointer access.

With this commit, 'dm_test_eth_act' test case passes.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>