iwlwifi: check endianness annotations by default

sparse won't check endianness annotations by
default, but iwlwifi is and should be clean
so we can make sparse check them on it.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>