MIPv6: simplify fixed point degrees conversion

* Simplify the conversion of degrees from 24 bit fixed point to a float.
  Avoids strict-aliasing warning by gcc 4.4.
* Add reference to RFC 6757.
* Correct spelling of 'convert' in degrees_convert_fixed_to_float().

Change-Id: Icb680d009fdd960e8668fa3020060799d6a74c2f
Reviewed-on: https://code.wireshark.org/review/5913
Reviewed-by: Alexis La Goutte <alexis.lagoutte@gmail.com>
Petri-Dish: Alexis La Goutte <alexis.lagoutte@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Michael Mann <mmann78@netscape.net>