wifi: iwlwifi: mvm: refactor iwl_mvm_add_sta(), iwl_mvm_rm_sta()

As we have a new MLD STA cmd, there will be a different function to
add/remove a station in MLD mode. But both functions will share a
common part. Put this part in a separate function which will be used
later in adding/removing a sta in MLD mode.

Signed-off-by: Miri Korenblit <miriam.rachel.korenblit@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230320122330.d01e0c9ccdc3.I5e9e27c3b363b36209a0ff960d2e59708e7ff0bf@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>