net: wireless: Convert to use the preferred fallthrough macro

Convert the uses of fallthrough comments to fallthrough macro.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Link: https://lore.kernel.org/r/20200822082323.45495-1-linmiaohe@huawei.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>