brcmfmac: introduce brcmf_net_detach() function

In case of error during brcmf_bus_start() the network interfaces were
freed using free_netdev(). However, the interfaces may have additional
memory allocated which is not freed. The netdev has destructor set to
brcmf_cfg80211_free_netdev() which frees the additional memory if
allocated and call free_netdev(). The brcmf_net_detach() either calls
brcmf_cfg80211_free_netdev() directly or uses unregister_netdev() when
struct net_device::reg_state indicates the netdev was registered.

Reported-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>