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>
diff --git a/drivers/power/Kconfig b/drivers/power/Kconfig
index 421770d..0f11a0f 100644
--- a/drivers/power/Kconfig
+++ b/drivers/power/Kconfig
@@ -309,6 +309,7 @@
 config CHARGER_ISP1704
 	tristate "ISP1704 USB Charger Detection"
 	depends on USB_PHY
+	depends on USB_GADGET || !USB_GADGET # if USB_GADGET=m, this can't be 'y'
 	help
 	  Say Y to enable support for USB Charger Detection with
 	  ISP1707/ISP1704 USB transceivers.