usb: musb: fix Kconfig

After 62285963 (usb: musb: drop a gigantic amount of ifdeferry):

 - USB_GADGET_MUSB_HDRC is no longer selectable because it
   depends on the removed USB_MUSB_PERIPHERAL and USB_MUSB_OTG
   options

 - The Kconfig comment still says "Enable Host or Gadget support
   to see Inventra options", even though you now need to enable
   both of them to see Inventra options.

Fix the dependency and drop the anyway unnecessary comment.

Signed-off-by: Rabin Vincent <rabin@rab.in>
Signed-off-by: Felipe Balbi <balbi@ti.com>