firmware: samsung: acpm: Fix cross-thread RX length corruption

Sashiko identified a cross-thread RX length corruption bug when
reviewing the thermal addition to ACPM [1].

When multiple threads concurrently send IPC requests, the ACPM polling
mechanism can encounter responses belonging to other threads. To drain
the queue, the driver saves these concurrent responses into an internal
cache (`rx_data->cmd`) to be retrieved later by the owning thread.

Previously, the driver incorrectly used `xfer->rxcnt` (the expected
receive length of the *current* polling thread) when copying data for
*other* threads into this cache. If the threads expected responses of
different lengths, this resulted in buffer underflows (leading to reads
of uninitialized memory) or potential buffer overflows.

Fix this by replacing the boolean `response` flag in
`struct acpm_rx_data` with `rxcnt`, caching the exact expected receive
length for each specific transaction during transfer preparation. Use
this cached length when saving concurrent responses.

Consequently, ensure that `xfer->rxcnt` is explicitly zeroed in driver
helpers (e.g., `acpm_dvfs_set_xfer`) for fire-and-forget messages to
prevent uninitialized stack garbage from being interpreted as a massive
expected receive length.

Cc: stable@vger.kernel.org
Fixes: a88927b534ba ("firmware: add Exynos ACPM protocol driver")
Closes: https://sashiko.dev/#/patchset/20260420-acpm-tmu-v3-0-3dc8e93f0b26%40linaro.org [1]
Reported-by: Titouan Ameline de Cadeville <titouan.ameline@gmail.com>
Closes: https://lore.kernel.org/r/20260426210255.73674-1-titouan.ameline@gmail.com/
Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Link: https://patch.msgid.link/20260505-acpm-fixes-sashiko-reports-v5-1-43b5ee7f1674@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>

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