brcmfmac: flush queue upon MACDESC_DEL firmware signal

When firmware signals the driver to remove a destination entry
it may have sk_buff packets queued for it. These should be freed.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
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>