usb: USB hubs require host mode

USB hubs run in host mode not in gadget mode. Hence, compiling usb_hub.c
should not be selected by CONFIG_USB_GADGET.

Suggested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Marek Vasut <marex@denx.de>