tty/serial: digicolor: fix possible null-ptr-deref in digicolor_uart_probe()

It will cause null-ptr-deref when using 'res', if platform_get_resource()
returns NULL, so move using 'res' after devm_ioremap_resource() that
will check it to avoid null-ptr-deref.
And use devm_platform_get_and_ioremap_resource() to simplify code.

Fixes: 5930cb35

 ("serial: driver for Conexant Digicolor USART")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Baruch Siach <baruch@tkos.co.il>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20220505124621.1592697-1-yangyingliang@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>