kernel: move CONFIG_USB_MTU3 to generic config

CONFIG_USB_MTU3 is not visible for the mediatek target by default, but
only when CONFIG_USB_GADGET is set. This will config option will be
remove with when running "make kernel_oldconfig", move this option to
the generic config to prevent this.

This fixes the build of the mt7623 subtarget of the mediatek target.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

target/linux/generic/config-4.14

@@ -5201,6 +5201,7 @@ CONFIG_USB_GADGET_VBUS_DRAW=2
 # CONFIG_USB_MON is not set
 # CONFIG_USB_MOUSE is not set
 # CONFIG_USB_MSM_OTG is not set
+# CONFIG_USB_MTU3 is not set
 # CONFIG_USB_MUSB_HDRC is not set
 # CONFIG_USB_MV_U3D is not set
 # CONFIG_USB_MV_UDC is not set

target/linux/mediatek/mt7622/config-4.14

@@ -444,7 +444,6 @@ CONFIG_TREE_SRCU=y
 CONFIG_USB=y
 CONFIG_USB_COMMON=y
 # CONFIG_USB_EHCI_HCD is not set
-# CONFIG_USB_MTU3 is not set
 CONFIG_USB_SUPPORT=y
 CONFIG_USB_XHCI_HCD=y
 CONFIG_USB_XHCI_MTK=y