remoteproc: mediatek: Zero out only remaining bytes of IPI buffer

In scp_ipi_handler(), instead of zeroing out the entire shared
buffer, which may be as large as 600 bytes, overwrite it with the
received data, then zero out only the remaining bytes.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20240520112724.139945-1-angelogioacchino.delregno@collabora.com


Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>