mac80211: Don't squash error codes in key setup functions

ieee80211_add_key() currently returns -ENOMEM in case of any error,
including a missing crypto algorithm.  Change ieee80211_key_alloc()
and ieee80211_aes_{key_setup_encrypt,cmac_key_setup}() to encode
errors with ERR_PTR() rather than returning NULL, and change
ieee80211_add_key() accordingly.

Compile-tested only.

Reported-by: Marcin Owsiany <porridge@debian.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/net/mac80211/aes_ccm.c b/net/mac80211/aes_ccm.c
index a87cb3b..d2b03e0 100644
--- a/net/mac80211/aes_ccm.c
+++ b/net/mac80211/aes_ccm.c
@@ -138,10 +138,8 @@
 	struct crypto_cipher *tfm;
 
 	tfm = crypto_alloc_cipher("aes", 0, CRYPTO_ALG_ASYNC);
-	if (IS_ERR(tfm))
-		return NULL;
-
-	crypto_cipher_setkey(tfm, key, ALG_CCMP_KEY_LEN);
+	if (!IS_ERR(tfm))
+		crypto_cipher_setkey(tfm, key, ALG_CCMP_KEY_LEN);
 
 	return tfm;
 }