mac80211_hwsim: use DEFINE_DEBUGFS_ATTRIBUTE to define debugfs fops

It is more clear to use DEFINE_DEBUGFS_ATTRIBUTE to define debugfs file
operation rather than DEFINE_SIMPLE_ATTRIBUTE.

It is detected with the help of coccinelle.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Link: https://lore.kernel.org/r/1572404462-45462-1-git-send-email-zhongjiang@huawei.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>