cfg80211: fix a crash in dev lookup on dump commands

IS_ERR and PTR_ERR were called with the wrong pointer, leading to a
crash when cfg80211_get_dev_from_ifindex fails.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>