mips/float_dsp: fix vector_fmul_window_mips on mips64

Commit dfa920807494 ("mips/float_dsp: fix a bug in vector_fmul_window_mips")
fixed vector_fmul_window_mips by unrolling the loop only 4 times, but also
removed the outer C loop and replaced it with assembly branches and pointer
arithmetic. When submitting my 64-bit porting patch I missed this new
assembly which also needed porting.

This patch fixes a bus error in the fate-float-dsp test when run on 64-bit
mips.

Signed-off-by: James Cowgill <james410@cowgill.org.uk>
Reviewed-by: Nedeljko Babic <nedeljko.babic@imgtec.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
