brcmfmac: check memory allocation in brcmf_add_if()

For P2P_DEVICE interface the struct brcmf_if instance is
allocated using kzalloc() which can fail. Add pointer
check and return -ENOMEM if it failed. Fixes the following
smatch error:

"drivers/net/wireless/brcm80211/brcmfmac/dhd_linux.c:770
brcmf_add_if()
  error: potential null dereference 'ifp'. (kzalloc returns null)"

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

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

@@ -754,6 +754,8 @@ struct brcmf_if *brcmf_add_if(struct brcmf_pub *drvr, s32 bssidx, s32 ifidx,
 		/* this is P2P_DEVICE interface */
 		brcmf_dbg(INFO, "allocate non-netdev interface\n");
 		ifp = kzalloc(sizeof(*ifp), GFP_KERNEL);
+		if (!ifp)
+			return ERR_PTR(-ENOMEM);
 	} else {
 		brcmf_dbg(INFO, "allocate netdev interface\n");
 		/* Allocate netdev, including space for private structure */