mac80211: fils_aead: fix encrypt error handling

gcc -Wmaybe-uninitialized reports a bug in aes_siv_encryp:

net/mac80211/fils_aead.c: In function ‘aes_siv_encrypt.constprop’:
net/mac80211/fils_aead.c:84:26: error: ‘tfm2’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

At the time that the memory allocation fails, 'tfm2' has not been
allocated, so we should not attempt to free it later, and we can
simply return an error.

Fixes: 39404fee ("mac80211: FILS AEAD protection for station mode association frames")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>