mfd: sec-common: Use sizeof(*var), not sizeof(struct type_of_var)

Using sizeof(*var) is generally preferred over using the size of its
open-coded type when allocating memory.

This helps avoiding bugs when the variable type changes but the memory
allocation isn't updated, and it simplifies renaming of the struct if
ever necessary.

No functional change.

Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/r/20250409-s2mpg10-v4-20-d66d5f39b6bf@linaro.org


Signed-off-by: Lee Jones <lee@kernel.org>