qtnfmac: check that MAC exists in regulatory notifier

It is possible that regulatory notifier is called before MAC data
was allocated. We need to verify that MAC data exists before trying
to send a regulatory change event.

Signed-off-by: Igor Mitsyanko <igor.mitsyanko.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>