Address review: narrow CryptoCb introspection to a boolean check

- Replace public wc_CryptoCb_GetDevice() with
  wc_CryptoCb_IsDeviceRegistered() returns 1 or 0. keep the CryptoCb
  struct and GetDevice private.
- Reject RegisterDevice(INVALID_DEVID) with BAD_FUNC_ARG instead of ALREADY_E.
- Document the new API and the ALREADY_E/BAD_FUNC_ARG returns.
- Fix table-full test to not leak when MAX_CRYPTO_DEVID_CALLBACKS >= 256.

wolfcrypt/src/cryptocb.c

@@ -80,10 +80,12 @@ Crypto Callback Build Options:
     #include <wolfssl/wolfcrypt/port/caam/wolfcaam.h>
 #endif
 /* TODO: Consider linked list with mutex */
-#ifndef MAX_CRYPTO_DEVID_CALLBACKS
-#define MAX_CRYPTO_DEVID_CALLBACKS 8
-#endif
 
+typedef struct CryptoCb {
+    int devId;
+    CryptoDevCallbackFunc cb;
+    void* ctx;
+} CryptoCb;
 static WC_THREADSHARED CryptoCb gCryptoDev[MAX_CRYPTO_DEVID_CALLBACKS];
 
 #ifdef WOLF_CRYPTO_CB_FIND
@@ -350,7 +352,7 @@ void wc_CryptoCb_InfoString(wc_CryptoInfo* info)
 
 /* Search through listed devices and return the first matching device ID
  * found. */
-CryptoCb* wc_CryptoCb_GetDevice(int devId)
+static CryptoCb* wc_CryptoCb_GetDevice(int devId)
 {
     int i;
     for (i = 0; i < MAX_CRYPTO_DEVID_CALLBACKS; i++) {
@@ -360,6 +362,15 @@ CryptoCb* wc_CryptoCb_GetDevice(int devId)
     return NULL;
 }
 
+/* Returns 1 if the given device ID is currently registered, 0 otherwise.
+ * INVALID_DEVID marks free table slots, so it is never reported registered. */
+int wc_CryptoCb_IsDeviceRegistered(int devId)
+{
+    if (devId == INVALID_DEVID)
+        return 0;
+    return wc_CryptoCb_GetDevice(devId) != NULL;
+}
+
 
 /* Filters through find callback set when trying to get the device,
  * returns the device found on success and null if not found. */
@@ -448,6 +459,10 @@ int wc_CryptoCb_RegisterDevice(int devId, CryptoDevCallbackFunc cb, void* ctx)
     int rc = 0;
     CryptoCb* dev;
 
+    /* INVALID_DEVID marks a free slot and cannot be registered as a device. */
+    if (devId == INVALID_DEVID)
+        return BAD_FUNC_ARG;
+
     /* Reject re-registration of an already-registered device ID. */
     if (wc_CryptoCb_GetDevice(devId) != NULL)
         return ALREADY_E;