ASoC: SOF: ipc-msg-injector: Propagate write errors correctly

This code is supposed to propagate errors from simple_write_to_buffer()
or return -EFAULT if "size != count".  However "size" needs to be signed
for the code to work correctly and the case where "size == 0" is not
handled correctly.

Fixes: 066c6762 ("ASoC: SOF: ipc-msg-injector: Add support for IPC4 messages")
Fixes: 2f0b1b01 ("ASoC: SOF: debug: Add support for IPC message injection")
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+Cd+JrfOH0i7z@kili


Signed-off-by: Mark Brown <broonie@kernel.org>