lib: utils/serial: Skip baudrate config if input frequency is zero

We should skip baudrate config for UART8250 and SiFive UART when
input frequency is zero.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>