media: venus: hfi_cmds: Fix packet size calculation

Now that a one-element array was replaced with a flexible-array member
in struct hfi_sys_set_property_pkt, use the struct_size() helper to
correctly calculate the packet size.

Fixes: 701e10b3fd9f ("media: venus: hfi_cmds.h: Replace one-element array with flexible-array member")
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Stanimir Varbanov <stanimir.varbanov@linaro.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>