usb: Do not compile USB_STORAGE with BLK && !DM_USB

The USB storage driver does not compile when CONFIG_BLK is set,
but DM_USB is not set, as we're missing the DM device links for
CONFIG_BLK enabled code paths.

So far it looks like nobody fell into this trap, because no board
enabled CONFIG_BLK and CONFIG_USB_STORAGE while not enabling
CONFIG_DM_USB, but we should still reflect that dependency properly
in Kconfig so that implicit enabling of CONFIG_USB_STORAGE works.

Signed-off-by: Alexander Graf <agraf@suse.de>