iwlwifi: mvm: only send LEDS_CMD when the FW supports it

The LEDS_CMD command is only supported in some newer FW versions
(e.g. iwlwifi-8000C-31.ucode), so we can't send it to older versions
(such as iwlwifi-8000C-27.ucode).

To fix this, check for a new bit in the FW capabilities TLV that tells
when the command is supported.

Note that the current version of -31.ucode in linux-firmware.git
(31.532993.0) does not have this capability bit set, so the LED won't
work, even though this version should support it.  But we will update
this firmware soon, so it won't be a problem anymore.

Fixes: 7089ae63

 ("iwlwifi: mvm: use firmware LED command where applicable")
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>