brcmfmac: make brcmf_p2p_detach() call conditional

During verification of error handling in brcmf_cfg80211_attach() a
null pointer dereference occurred upon calling brcmf_p2p_detach()
from brcmf_detach(). This should only be called when the
brcmf_cfg80211_attach() has succeeded.

Fixes: f7a40873 ("brcmfmac: assure p2pdev is unregistered upon driver unload")
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>