firmware: samsung: acpm: Annotate rx_data->cmd with __counted_by_ptr

firmware: samsung: acpm: Annotate rx_data->cmd with __counted_by_ptr

Rename the `n_cmd` member of `struct acpm_rx_data` to `cmdcnt` to
maintain consistent nomenclature across the driver (aligning with
`txcnt`, `rxcnt`, and transfer helpers).

With the member renamed, annotate the dynamically allocated `cmd`
pointer with the `__counted_by_ptr(cmdcnt)` macro to improve runtime
bounds checking.

Signed-off-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Reviewed-by: Peter Griffin <peter.griffin@linaro.org>
Link: https://patch.msgid.link/20260515-acpm-tmu-helpers-v2-2-8ca011d5a965@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>