wifi: iwlwifi: mvm: fix potential array out of bounds access

Account for IWL_SEC_WEP_KEY_OFFSET when needed while verifying
key_len size in iwl_mvm_sec_key_add().

Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230613155501.f193b7493a93.I6948ba625b9318924b96a5e22602ac75d2bd0125@changeid


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