ASoC: SOF: ipc-msg-injector: Fix reversed if statement

This if statement is reversed.  In fact, the condition can just be
deleted because writing zero bytes is a no-op.

Fixes: 066c6762

 ("ASoC: SOF: ipc-msg-injector: Add support for IPC4 messages")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@linux.intel.com>
Link: https://lore.kernel.org/r/Yph+T3PpGCdPsEDj@kili
Signed-off-by: Mark Brown <broonie@kernel.org>