spandsp: disable fixed point implementation

spandsp: disable fixed point implementation

On a Netgear R7000 (ARMv7) without floating point, using SpanDSP with
fixed point to send/receive fax on Asterisk resulted in repeated
failure and intermittent performance.  Disabling fixed point fixed
these issues; the software floating point implementation was fine for
real-time use.

Now on a Netgear R7800 (ARMv7) with hardware floating point, the
autoconf auto-detect will say no floating point is available, and
again enables the poor performing fixed point implementation.  Ouch!