brcmfmac: use struct net_device::destructor to remove interfaces

Upon deleting a P2P_CLIENT/GO interface the vif and consequently
the wdev is freed before the net_device is actually being unregistered
but cfg80211 still needs to access the wdev. Using destructor field
to free the net_device and vif.

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>