ice: use flex_array_size where possible

Use the flex_array_size() helper with the recently added flexible array
members in structures.

Signed-off-by: Bruce Allan <bruce.w.allan@intel.com>
Tested-by: Tony Brelinski <tonyx.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>