shell: telnet: Make sure the delayed work becomes idle after canceling

Replace k_work_cancel_delayable() with k_work_cancel_delayable_sync()
to make sure that the submitted work becomes idle before accessing
sensitive data.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>

include/shell/shell_telnet.h

@@ -46,6 +46,7 @@ struct shell_telnet {
 	 *  to send the shell prompt for instance.
 	 */
 	struct k_work_delayable send_work;
+	struct k_work_sync work_sync;
 
 	/** If set, no output is sent to the TELNET client. */
 	bool output_lock;