hwspinlock: stm32: Use struct_size() helper in devm_kzalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/hwspinlock/stm32_hwspinlock.c:84:32: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174


Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang7@gmail.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220125021353.GA29777@embeddedor