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

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

This patch converts the f_tcm.c to the new function interface.

The file can be now compiled into a separate module usb_f_tcm.ko.

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>
Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/usb/gadget/legacy/tcm_usb_gadget.c

@@ -24,6 +24,7 @@
 USB_GADGET_COMPOSITE_OPTIONS();
 
 /* #include to be removed when new function registration interface is used  */
+#define USBF_TCM_INCLUDED
 #include "../function/f_tcm.c"
 
 #define UAS_VENDOR_ID	0x0525	/* NetChip */