batman-adv: rename equiv/equal or better to similar or better

Since the function applies a threshold and also slightly worse
values are accepted, ''equal or better'' does not represent the
intention of the function. ''Similar or better'' represents that better.

Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Signed-off-by: Antonio Quartulli <antonio@meshcoding.com>