test_firmware: make local symbol test_fw_config static

Fixes the following sparse warnings:

lib/test_firmware.c:99:20: warning:
 symbol 'test_fw_config' was not declared. Should it be static?
Signed-off-by: NWei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/test_firmware.c

@@ -96,7 +96,7 @@ struct test_config {
 			    struct device *device);
 };
 
-struct test_config *test_fw_config;
+static struct test_config *test_fw_config;
 
 static ssize_t test_fw_misc_read(struct file *f, char __user *buf,
 				 size_t size, loff_t *offset)