tty: hvc: make symbol 'hvc_udbg_dev' static

The sparse tool complains as follows:

drivers/tty/hvc/hvc_udbg.c:20:19: warning:
 symbol 'hvc_udbg_dev' was not declared. Should it be static?

This symbol is not used outside of hvc_udbg.c, so this
commit marks it static.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Link: https://lore.kernel.org/r/20210407125826.4139130-1-yukuai3@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/hvc/hvc_udbg.c

@@ -17,7 +17,7 @@
 
 #include "hvc_console.h"
 
-struct hvc_struct *hvc_udbg_dev;
+static struct hvc_struct *hvc_udbg_dev;
 
 static int hvc_udbg_put(uint32_t vtermno, const char *buf, int count)
 {