Rename plain c option and update comments

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/aesni.c b/library/aesni.c
index 31321c4..cc3a3b3 100644
--- a/library/aesni.c
+++ b/library/aesni.c
@@ -39,7 +39,7 @@
 #include <immintrin.h>
 #endif
 
-#if !defined(MBEDTLS_AES_DONT_USE_SOFTWARE_CRYPTO)
+#if !defined(MBEDTLS_AES_USE_HARDWARE_ONLY)
 /*
  * AES-NI support detection routine
  */
@@ -69,7 +69,7 @@
 
     return (c & what) != 0;
 }
-#endif /* !MBEDTLS_AES_DONT_USE_SOFTWARE_CRYPTO */
+#endif /* !MBEDTLS_AES_USE_HARDWARE_ONLY */
 
 #if MBEDTLS_AESNI_HAVE_CODE == 2