treewide: Test on CONFIG_USB_HOST (or CONFIG_CMD_USB) not CONFIG_USB

As the logic here is only used when we have a USB host controller, test
on CONFIG_USB_HOST rather than CONFIG_USB in general.  This lets us move
towards using CONFIG_USB only as a menu symbol to say that we have some
form of USB, and then USB_HOST or USB_GADGET depending on the role that
USB plays within the build.

Signed-off-by: Tom Rini <trini@konsulko.com>