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>

common/cmd_bdinfo.c

@@ -36,6 +36,7 @@ static void print_num(const char *name, ulong value)
 	printf("%-12s= 0x%08lX\n", name, value);
 }
 
+__maybe_unused
 static void print_eth(int idx)
 {
 	char name[10], *val;