tty: hvc: make hvc_rtas_dev static

The symbol is not used outside of the file, so mark it static.

Fixes the following warning:

drivers/tty/hvc/hvc_rtas.c:29:19: warning: symbol 'hvc_rtas_dev' was
not declared. Should it be static?
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: ruanjinjie <ruanjinjie@huawei.com>
Link: https://lore.kernel.org/r/20221019064412.3759874-1-ruanjinjie@huawei.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/hvc/hvc_rtas.c

@@ -26,7 +26,7 @@
 #include "hvc_console.h"
 
 #define hvc_rtas_cookie 0x67781e15
-struct hvc_struct *hvc_rtas_dev;
+static struct hvc_struct *hvc_rtas_dev;
 
 static int rtascons_put_char_token = RTAS_UNKNOWN_SERVICE;
 static int rtascons_get_char_token = RTAS_UNKNOWN_SERVICE;