usb: gadget: f_obex: convert to new function interface with backward compatibility

Converting f_obex to the new function interface requires converting
the f_obex's function code and its users.

This patch converts the f_obex.c to the new function interface.
The file is now compiled into a separate usb_f_obex.ko module.

The old function interface is provided by means of preprocessor
conditional directives. After all users are converted, the old interface
can be removed.

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/Makefile b/drivers/usb/gadget/Makefile
index 34bb49a..6afd166 100644
--- a/drivers/usb/gadget/Makefile
+++ b/drivers/usb/gadget/Makefile
@@ -43,6 +43,8 @@
 obj-$(CONFIG_USB_U_SERIAL)	+= u_serial.o
 usb_f_serial-y			:= f_serial.o
 obj-$(CONFIG_USB_F_SERIAL)	+= usb_f_serial.o
+usb_f_obex-y			:= f_obex.o
+obj-$(CONFIG_USB_F_OBEX)	+= usb_f_obex.o
 
 #
 # USB gadget drivers