calculateCrossFadeChunks() still returns int, although the caller uses
it as an unsigned value.  Since the function body checks for negative
values, it is safe to cast to unsigned.

crossFade() takes signed parameters, although it callers pass unsigned
integers.  Change declaration to unsigned.

git-svn-id: https://svn.musicpd.org/mpd/trunk@7291 09075e82-0dd4-0310-85a5-a0d7c8717e4f