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>