usb: dwc3: Fix typo 'USB_CONFIG' to 'CONFIG_USB'

Commit deca2724 ("USB: Fix compilation errors with
host stack disabled") was added for checking if USB_CONFIG
was enable only then go for registering the host notifier.
However, this should be CONFIG_USB instead of that.
Since USB_CONFIG is not defined anywhere it will take it
false and the host notifier will never be registered.

Fix this by correcting the typo to CONFIG_USB.

Change-Id: I99b2224e1973116e3001a5fbb5e1a569bdae2105
Signed-off-by: Udipto Goswami <ugoswami@codeaurora.org>