nl80211: nl80211hdr_put() doesn't return an ERR_PTR

There are a few places which check nl80211hdr_put() for an ERR_PTR
but actually it returns NULL on error and never error values.  In
nl80211_testmode_dump() the return wasn't checked at all so I have
added one.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
[some whitespace changes]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>