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

hwspinlock: sprd: 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/sprd_hwspinlock.c:96:36: warning: using sizeof on a flexible structure

Link: https://github.com/KSPP/linux/issues/174
Reviewed-by: Baolin Wang <baolin.wang7@gmail.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>