iwlwifi: mvm: rs: don't clear persistent fields

iwl_mvm_rs_rate_init() is called multiple times to re-init
the rate scaling statistics (e.g. after some idle time).

It clears all the lq_sta sta, including some fields that
shouldn't be cleared (e.g. debugfs pointers). Fix it
by adding a new 'persistent' sub-struct, and
avoid clearing it on (re-)init.

Move the initialization of the persistent fields to
rs_alloc_sta instead.

Signed-off-by: Eliad Peller <eliadx.peller@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>