iwlwifi: mvm: add debugfs entry for ps_disabled

In order to make debugging easier, add an entry to export the
ps_disabled value via debugfs.  To make usage of the
debugfs_create_*() function easier, change the ps_disabled element to
u8.

Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>