sound: avoid endless loop

'sound play 1 100000' results in an endless loop on the sandbox.

If the frequency exceeds half the sampling rate, zero out the output
buffer.

Fixes: 511ed5fd ("SOUND: SAMSUNG: Add I2S driver")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>