usb: gadget: Potential NULL dereference on allocation error

The source_sink_alloc_func() function is supposed to return error
pointers on error.  The function is called from usb_get_function() which
doesn't check for NULL returns so it would result in an Oops.

Of course, in the current kernel, small allocations always succeed so
this doesn't affect runtime.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/f_sourcesink.c

@@ -838,7 +838,7 @@ static struct usb_function *source_sink_alloc_func(
 
 	ss = kzalloc(sizeof(*ss), GFP_KERNEL);
 	if (!ss)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	ss_opts =  container_of(fi, struct f_ss_opts, func_inst);