wifi: iwlwifi: mvm: factor out iwl_mvm_sta_fw_id_mask()

wifi: iwlwifi: mvm: factor out iwl_mvm_sta_fw_id_mask()

We are going to need this in more places than just the
key code, so factor out the functionality of getting
the FW station ID mask (filtered to a specific link if
needed) to a separate function that can now be called
both under RCU and mvm->mutex protection.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230416154301.eff92b93025d.I2c50290a0537d5db3d3460f4d57c78a4712ffb75@changeid
Signed-off-by: Johannes Berg <johannes.berg@intel.com>