lib: utils/serial: Initialize platform_uart_data to zero

While it doesn't look like there are any current cases of using
uninitialized data, let's zero all the UART data members to be
safe. Zero may not actually be better than a random number in
some cases, so all structure members should still be validated
before use, but at least zero is usually easier to debug than
some random stack garbage...

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>