wifi: iwlwifi: mvm: initialize per-link STA ratescale data

When allocating a new link station, initialize the ratescaling
data for it. To do that, refactor the initialization code out
into a new iwl_mvm_rs_add_sta_link() function.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230417113648.441b6a8c60fd.I34f1b3555c25aaa22cc34d1112fc3b6393a20b7c@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>