Specify maximum sample rate of MLP by defining the factor relative to 48000

Specify maximum sample rate of MLP by defining the factor relative to 48000
instead of directly.
This makes clear that the code assumes the maximum sample rate to be
a multiple of 48000 and also removes the division from the MAX_BLOCKSIZE
macros, which causes an issue with the Solaris assembler where "/" is
a comment marker unless the --divide option is used.

Originally committed as revision 20026 to svn://svn.ffmpeg.org/ffmpeg/trunk