net: Only access network devices after init

In the efi_loader main loop we call eth_rx() occasionally. This rx function
might end up calling into devices that haven't been initialized yet,
potentially resulting in a lot of transfer timeouts.

Instead, let's make sure the ethernet device is actually initialized before
reading from or writing to it.

Signed-off-by: Alexander Graf <agraf@suse.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>