FFMPEG Plugin: Code styles, typos and code deduplication.

- Kudos to stippi for taking the time pointing those out :)
  http://www.freelists.org/post/haiku-commits/haiku-hrev47576-srcaddonsmediapluginsffmpeg,1
- No functional change intended.

Signed-off-by: Colin Günther <coling@gmx.de>
(cherry picked from commit e813febdba487833b90460871b0c5b099c44f6b9)