brcmfmac: use custom destructor callback for all netdevice interfaces

The destructor for net devices was set to free_netdev() to get rid
of it and the private data. The private data refers to a brcmf_if
instance, but indirectly it also refers to brcmf_cfg80211_vif which
holds the wdev. This is freed as well by using a new custom destructor
called brcmf_cfg80211_free_netdev().

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