ice: switch: add and use u16[] aliases to ice_adv_lkup_elem::{h, m}_u

ice_adv_lkup_elem fields h_u and m_u are being accessed as raw u16
arrays in several places.
To reduce cast and braces burden, add permanent array-of-u16 aliases
with the same size as the `union ice_prot_hdr` itself via anonymous
unions to the actual struct declaration, and just access them
directly.

This:
 - removes the need to cast the union to u16[] and then dereference
   it each time -> reduces the horizon for potential bugs;
 - improves -Warray-bounds coverage -- the array size is now known
   at compilation time;
 - addresses cppcheck complaints.

Signed-off-by: Alexander Lobakin <alexandr.lobakin@intel.com>
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Tested-by: Marcin Szycik <marcin.szycik@linux.intel.com>
Tested-by: Sandeep Penigalapati <sandeep.penigalapati@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>