mac802154: llsec: fix return value check in llsec_key_alloc()

In case of error, the functions crypto_alloc_aead() and crypto_alloc_blkcipher()
returns ERR_PTR() and never returns NULL. The NULL test in the return value check
should be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>