lfe-filter: Enable LFE filter in the resampler

lfe-filter: Enable LFE filter in the resampler

When enable-lfe-remixing is set, an LFE channel is present in the
resampler's destination channel map but not in the source channel map,
we insert a low-pass filter instead of just averaging the channels.
Other channels will get a high-pass filter.

In this patch, the crossover frequency is hardcoded to 120Hz (to be fixed
in later patches).

Note that in current state the LFE filter is
 - not very optimised
 - not rewind friendly (rewinding can cause audible artifacts)

Signed-off-by: David Henningsson <david.henningsson@canonical.com>