batman-adv: use htons when possible

When comparing a network ordered value with a constant, it
is better to convert the constant at compile time by means
of htons() instead of converting the value at runtime using
ntohs().

This refactoring may slightly improve the code performance.

Moreover substitute __constant_htons() with htons() since
the latter increase readability and it is smart enough to be
as efficient as the former

Signed-off-by: Antonio Quartulli <ordex@autistici.org>
Signed-off-by: Marek Lindner <lindner_marek@yahoo.de>
Acked-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>