[nrf fromtree] net: lib: tls_credentials: add invalid sec tag value

Add a definition for an invalid secure tag value to be used
as a placeholder. Negative values are reserved for internal use,
but some of them should be considered valid.
To be able to check against a value, we need to define an invalid one.

(cherry picked from commit 59a8eeb)

Signed-off-by: Maximilian Deubel <[email protected]>

Author: maxd-nordic

include/zephyr/net/tls_credentials.h

@@ -77,6 +77,8 @@ enum tls_credential_type {
  */
 typedef int sec_tag_t;
 
+#define SEC_TAG_TLS_INVALID (-1) /**< Invalid secure tag value. */
+
 /**
  * @brief Add a TLS credential.
  *