usb: Add missing guard around env_get() in usb_hub

The env_get() might be undefined in case ENV_SUPPORT is disabled,
which may happen e.g. in SPL. Add missing ifdef guard around the
env_get() to prevent build failure.

Signed-off-by: Marek Vasut <marex@denx.de>
Tested-by: Fabio Estevam <festevam@denx.de>
diff --git a/common/usb_hub.c b/common/usb_hub.c
index d736389..95f1449 100644
--- a/common/usb_hub.c
+++ b/common/usb_hub.c
@@ -168,7 +168,7 @@
 	int i;
 	struct usb_device *dev;
 	unsigned pgood_delay = hub->desc.bPwrOn2PwrGood * 2;
-	const char *env;
+	const char __maybe_unused *env;
 
 	dev = hub->pusb_dev;
 
@@ -193,10 +193,12 @@
 	 * but allow this time to be increased via env variable as some
 	 * devices break the spec and require longer warm-up times
 	 */
+#if CONFIG_IS_ENABLED(ENV_SUPPORT)
 	env = env_get("usb_pgood_delay");
 	if (env)
 		pgood_delay = max(pgood_delay,
 			          (unsigned)simple_strtol(env, NULL, 0));
+#endif
 	debug("pgood_delay=%dms\n", pgood_delay);
 
 	/*