iwlwifi: mvm: set wep key for all stations in soft ap mode

When operating as a soft ap with wep security, the key was not
configured to the fw for the stations, based on the fact that the
key will be specified in the tx command.

However, in the new tx api the tx command does not include the key,
which resulted in all data frames going out un-encrypted.

Fix it by configuring the key for all the stations as they are added.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>