bluetooth: ldac: Use format as FLOAT32LE

LDAC encoder already supports S16, S24, S32 and F32LE. Using FLOAT32LE
for the sample format would avoid the additional call for conversion to
pa_sconv_s32le_from_float32ne. perf tool shows this as being the function
called frequently after encode. So, just avoid this by using sample format
as F32LE.

Part-of: <pulseaudio/pulseaudio!486>