iwlwifi: mvm: use firmware LED command where applicable

On devices starting from 8000 series, the host can no longer toggle
the LED through the CSR_LED_REG register, but must do it via the
firmware instead. Add support for this. Note that this means that
the LED cannot be turned on while the firmware is off, so using an
arbitrary LED trigger may not work as expected.

Fixes: 503ab8c5 ("iwlwifi: Add 8000 HW family support")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>