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>