drivers/clk: convert VL struct to struct_size

There are a few manually-calculated variable-length struct allocations
left, this converts them to use struct_size. Found with the following
git grep command

	git grep -A1 'kzalloc.*sizeof[^_].*+'

Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lkml.kernel.org/r/20190927185110.29897-1-steve@sk2.org


Acked-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
[sboyd@kernel.org: Add grep command]
Signed-off-by: Stephen Boyd <sboyd@kernel.org>