change how we store the FIR coefficients

The coefficient table is now transposed and shows
much better its polyphase nature: we now have a FIR
per line, each line corresponding to a phase.

This doesn't change at all the results produced by
the filter, but allows us to make slightly better
use of the data cache and improves performance a bit
(although not as much as I thought it would).

The main benefit is that it is the first step
before we can make much larger optimizations
(like using NEON).

Change-Id: Iebf7695825dcbd41f25861efcaefbaa3365ecb43