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: f7a40873d2fa ("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>

drivers/net/wireless/brcm80211/brcmfmac/core.c

@@ -1098,7 +1098,8 @@ void brcmf_detach(struct device *dev)
 
 	/* stop firmware event handling */
 	brcmf_fweh_detach(drvr);
-	brcmf_p2p_detach(&drvr->config->p2p);
+	if (drvr->config)
+		brcmf_p2p_detach(&drvr->config->p2p);
 
 	brcmf_bus_change_state(bus_if, BRCMF_BUS_DOWN);