iwlwifi: align timestamp cancel with timestamp start

Canceling the periodic timestamp work should be
done in the opposite flow to where it was started.
This also prevents from sending the MARKER command
during the mac_stop flow - causing a false queue hang
(FW is no longer there to send a response).

Fixes: 93b167c1 ("iwlwifi: runtime: sync FW and host clocks for logs")
Signed-off-by: Shaul Triebitz <shaul.triebitz@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>