drivers/net: Fix ptr_ret.cocci warnings.

we can use PTR_ERR_OR_ZERO rather than if(IS_ERR(...)) + PTR_ERR
1. drivers/net/appletalk/ipddp.c
2. drivers/net/wireless/broadcom/brcm80211/brcmfmac/debug.c

Generated by: scripts/coccinelle/api/ptr_ret.cocci

Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>