lib: utils/serial: Round UART8250 baud rate divisor to nearest integer

Previously, it was rounded down and that gives suboptimal results when
non-standard clock sources or baud rates are used.

Signed-off-by: Jakub Luzny <jakub.luzny@codasip.com>
Reviewed-by: Anup Patel <anup@brainfault.org>