usb: gadget: storage: Add error handling for no memory

fsg_common_set_num_buffers() may fail due to ENOMEM. So add
error handling for fail case.
Acked-by: NAlan Stern <stern@rowland.harvard.edu>
Acked-by: NMichal Nazarewicz <mina86@mina86.com>
Signed-off-by: NJaejoong Kim <climbbb.kim@gmail.com>
Signed-off-by: NFelipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/function/f_mass_storage.c

@@ -3308,7 +3308,9 @@ static ssize_t fsg_opts_num_buffers_store(struct config_item *item,
 	if (ret)
 		goto end;
 
-	fsg_common_set_num_buffers(opts->common, num);
+	ret = fsg_common_set_num_buffers(opts->common, num);
+	if (ret)
+		goto end;
 	ret = len;
 
 end: