iwlwifi: mvm: fix a memory leak in iwl_mvm_mac_ctxt_beacon_changed

If beacon_inject_active is true, we will return without freeing
beacon.  Fid that by freeing it before returning.

Signed-off-by: Zhang Qilong <zhangqilong3@huawei.com>
[reworded the commit message]
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210802172232.d16206ca60fc.I9984a9b442c84814c307cee3213044e24d26f38a@changeid


Signed-off-by: Luca Coelho <luciano.coelho@intel.com>