brcmfmac: move wiphy_unregister() call to brcmf_cfg80211_detach()

The wiphy_unregister() call was done in brcmf_free_vif() when the
last interface was being removed. This is not the obvious place to
do that. This patch moves it to the brcmf_cfg80211_detach(). This
removes the need to keep count of interfaces.

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>