iwlwifi: dvm: handle zero brightness for wifi LED

In order to have the LED being OFF constantly when the
brightness is set to 0, we need to pass IWL_LED_SOLID to
iwl_led_cmd as the off parameter, otherwise the led will
stay on constantly.

This fixes
https://bugzilla.kernel.org/show_bug.cgi?id=110551



Signed-off-by: Hubert Tarasiuk <hubert.tarasiuk@gmail.com>
[reworked the commit message]
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>