serial: 8250: Use C99 array initializer & define UART_REG_UNMAPPED

Use C99 array initializer insteads of comments and make unmapped checks
more obvious.

Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220624205424.12686-5-ilpo.jarvinen@linux.intel.com


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