Fix unused function in cmd_bdinfo.c

It is fine to use __maybe_unused instead of #ifdef, but we also need one
for print_eth() since not all boards have Ethernet. This fixes this
warning:

cmd_bdinfo.c:39:13: warning: 'print_eth' defined but not used [-Wunused-function]

Signed-off-by: Simon Glass <sjg@chromium.org>