qtnfmac: convert to SPDX license identifiers

Replace textual license with SPDX-License-Identifier.
Add an SPDX-License-Identifier for the Makefile.

Signed-off-by: Sergey Matyukevich <sergey.matyukevich.os@quantenna.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>