ASoC: SOF: Fix incorrect use of sizeof in sof_ipc3_do_rx_work()

Here hdr is a pointer, and we should measure the size of
struct sof_ipc_cmd_hdr.

Fixes: 12c41c77 ("ASoC: SOF: Refactor rx function for fuzzing")
Signed-off-by: Xia Fukun <xiafukun@huawei.com>
Reviewed-by: Curtis Malainey <cujomalainey@chromium.org>
Link: https://lore.kernel.org/r/20230807075118.128122-1-xiafukun@huawei.com


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