sox: disable ssp when unavailable

It seems autodetection isn't good. Fixes:
http://autobuild.buildroot.net/results/885/885c7daec7b727d8a7cf47c7576f0e79d4a5ddf8/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>