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.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>