iwlwifi: mvm: take the required lock when clearing time event data

When receiving a session protection end notification, the time event
data is cleared without holding the required lock. Fix it.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20200306151128.a49846a634e4.Id1ada7c5a964f5e25f4d0eacc2c4b050015b46a2@changeid