lib: print_color_rate(): Fix formatting small rates in IEC mode

lib: print_color_rate(): Fix formatting small rates in IEC mode

ISO/IEC units are distinguished from the decadic ones by using a prefixes
like "Ki", "Mi" instead of "K" and "M". The current code inserts the letter
"i" after the decadic unit when in IEC mode. However it does so even when
the prefix is an empty string, formatting 1Kbit in IEC mode as "1000ibit".
Fix by omitting the letter if there is no prefix.

Signed-off-by: Petr Machata <me@pmachata.org>
Signed-off-by: David Ahern <dsahern@gmail.com>