brcmfmac: add brcmf_p2p_detach() call in brcmf_cfg80211_detach()

The function brcmf_p2p_detach() was only called in error flow of the
brcmf_cfg80211_attach() routine, but it also needs to be called
upon brcmf_cfg80211_detach().

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>