serial: core: Drop duplicate NULL check in uart_*shutdown()

The free_page(addr), which becomes free_pages(addr, 0) checks addr
against 0. No need to repeat this check in the callers.

Acked-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220204152808.10808-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>