iwlwifi: mvm: don't fail completely if led mode is not supported

Blink led mode is not supported by iwlmvm. This doesn't mean
that we should prevent any operation if it is selected by
the user.
Instead of failing without any notice to the user, fallback
to the default mode (RF mode) if the blink mode
is selected and print an error to inform the user.

Reported-by: Steven Haigh <netwiz@crc.id.au>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>