nl80211: fix VENDOR_CMD_RAW_DATA

Since ERR_PTR() is an inline, not a macro, just open-code it
here so it's usable as an initializer, fixing the build in
brcmfmac.

Reported-by: Arend Van Spriel <arend.vanspriel@broadcom.com>
Fixes: 901bb989

 ("nl80211: require and validate vendor command policy")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>