dsp: p_fir: Fix FIR filter implementation
dsp: p_fir: Fix FIR filter implementation

The new implementation follows the same idea as the previous one: simultaneously processes blocks of 8 samples to leverage vectorization. On an i7-3770 PC, this function is almost 4 times faster than the evaluating the output sample-by-sample (compiled with -O3). Also removed the reference to dbuf from the function description as it does not exist anymore (it was removed from the API in commit #1fa28ced55ebddb522cd383ce41bf0a792aff177).

Signed-off-by: Luiz Chamon <88a261efe38359fe9d3c657d10e85c86b766fe68@gmail.com>

parallella/pal,mateunho/pal,debug-de-su-ka/pal,aolofsson/pal,parallella/pal,eliteraspberries/pal,Adamszk/pal3,debug-de-su-ka/pal,8l/pal,debug-de-su-ka/pal,8l/pal,olajep/pal,mateunho/pal,aolofsson/pal,olajep/pal,mateunho/pal,eliteraspberries/pal,aolofsson/pal,Adamszk/pal3,parallella/pal,parallella/pal,olajep/pal,eliteraspberries/pal,mateunho/pal,parallella/pal,olajep/pal,8l/pal,8l/pal,debug-de-su-ka/pal,debug-de-su-ka/pal,eliteraspberries/pal,aolofsson/pal,eliteraspberries/pal,mateunho/pal,Adamszk/pal3,Adamszk/pal3