usb: gadget: move gadget API functions to udc-core

instead of defining all functions as static inlines,
let's move them to udc-core and export them with
EXPORT_SYMBOL_GPL, that way we can make sure that
only GPL drivers will use them.

As a side effect, it'll be nicer to add tracepoints
to the gadget API.

While at that, also fix Kconfig dependencies to
avoid randconfig build failures.

Acked-By: Sebastian Reichel <sre@kernel.org>
Acked-by: Peter Chen <peter.chen@nxp.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/phy/Kconfig

@@ -176,6 +176,7 @@ config TWL4030_USB
 	tristate "TWL4030 USB Transceiver Driver"
 	depends on TWL4030_CORE && REGULATOR_TWL4030 && USB_MUSB_OMAP2PLUS
 	depends on USB_SUPPORT
+	depends on USB_GADGET || !USB_GADGET # if USB_GADGET=m, this can't 'y'
 	select GENERIC_PHY
 	select USB_PHY
 	help