gpio: 74x164: Annotate buffer with __counted_by()

gpio: 74x164: Annotate buffer with __counted_by()

Add the __counted_by() compiler attribute to the flexible array member
volumes to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Use struct_size() instead of manually calculating the number of bytes to
allocate the private structure with a buffer.

Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20250207151825.2122419-4-andriy.shevchenko@linux.intel.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>