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>