USB: core: Add input prompt and help text for USB_OTG config

bd6882 commit (usb: gadget: fix Kconfig warning) removes
the duplicate USB_OTG config from gadget/Kconfig.  But
does not copy the input prompt and help text to the original
config defined in core/Kconfig. Add them now.

Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>