aes.c: in linuxkm clause of WOLFSSL_AES_DIRECT section, gate on...

aes.c: in linuxkm clause of WOLFSSL_AES_DIRECT section, gate on WOLFSSL_LINUXKM && WOLFSSL_AESNI (with no-asm falling through to the generic definition), and use wolfssl-idiomatic WARN_UNUSED_RESULT, not linux-kernel-idiomatic __must_check.  (thanks to John S peer review)