usb: gadget: f_subset: add configfs support

f_subset learns about configfs so we can, eventually,
remove in-kernel gadget drivers.

Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
diff --git a/drivers/usb/gadget/Kconfig b/drivers/usb/gadget/Kconfig
index 0a444f3..3a72b9d 100644
--- a/drivers/usb/gadget/Kconfig
+++ b/drivers/usb/gadget/Kconfig
@@ -547,6 +547,16 @@
 
 # this first set of drivers all depend on bulk-capable hardware.
 
+config USB_CONFIGFS_ECM_SUBSET
+	boolean "Ethernet Control Model (CDC ECM) subset"
+	depends on USB_CONFIGFS
+	depends on NET
+	select USB_U_ETHER
+	select USB_F_SUBSET
+	help
+	  On hardware that can't implement the full protocol,
+	  a simple CDC subset is used, placing fewer demands on USB.
+
 config USB_CONFIGFS_EEM
 	bool "Ethernet Emulation Model (EEM)"
 	depends on USB_CONFIGFS