iwlwifi: remove debugfs entries before cfg80211

This patch solves a bug in debugfs unregistration. iwlwifi registers its
debugfs files under cfg80211's dir, but removed its files only after having
unregistered mac80211 that removes cfg80211 debugfs dir. This garbaged
debugfs and didn't allow to rmmod and insmod iwlwifi several times in a
row.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>