brcmfmac: consolidate mac_descriptor related function names

Just cleaning up and being consistent in naming operations
related to struct brcmf_fws_mac_descriptor objects.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>