ice: switch: unobscurify bitops loop in ice_fill_adv_dummy_packet()

A loop performing header modification according to the provided mask
in ice_fill_adv_dummy_packet() is very cryptic (and error-prone).
Replace two identical cast-deferences with a variable. Replace three
struct-member-array-accesses with a variable. Invert the condition,
reduce the indentation by one -> eliminate line wraps.

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>