iwlwifi: disallow log_event access if interface down

'echo 1 > log_event' generates the bogus "MAC is in deep sleep"
or "Timeout waiting for hardware access" log messages when
the interface is down, we should just disallow accessing the
device through debugfs when it is down.

Signed-off-by: Richard A. Griffiths <richardx.a.griffiths@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>