Rename plain c option and update comments

Signed-off-by: Jerry Yu <jerry.h.yu@arm.com>
diff --git a/library/padlock.c b/library/padlock.c
index 82b84bf..0011722 100644
--- a/library/padlock.c
+++ b/library/padlock.c
@@ -33,7 +33,7 @@
 
 #if defined(MBEDTLS_HAVE_X86)
 
-#if !defined(MBEDTLS_AES_DONT_USE_SOFTWARE_CRYPTO)
+#if !defined(MBEDTLS_AES_USE_HARDWARE_ONLY)
 /*
  * PadLock detection routine
  */