ANDROID: usb: gadget: Add missing inline qualifier to stub functions

When CONFIG_USB_GADGET is disabled:

In file included from drivers/usb/dwc2/core_intr.c:54:
In file included from drivers/usb/dwc2/core.h:43:
include/linux/usb/gadget.h:348:12: warning: unused function
'usb_gsi_ep_op' [-Wunused-function]
static int usb_gsi_ep_op(struct usb_ep *ep,
           ^
include/linux/usb/gadget.h:645:12: warning: unused function
'usb_gadget_func_wakeup' [-Wunused-function]
static int usb_gadget_func_wakeup(struct usb_gadget *gadget, int
interface_id)
           ^
2 warnings generated.

Stub functions are expected to use inline to hide the fact that they are
unused. Add that here to eliminate the warnings.

Change-Id: I55cabd7602e5ddf6213265053e1b7850ff62bcdb
Fixes: 632093ec ("ANDROID: GKI: USB: Resolve ABI diff for usb_gadget and usb_gadget_ops")
Fixes: b582ef5b ("ANDROID: GKI: drivers: usb: Add functions usb_func_ep_queue/usb_func_wakeup")
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>