dev/radio: fix doc about payload_len for prepare().
@@ -562,7 +562,7 @@ struct radio_driver {
* `payload` will contain the MAC header (MHR) and MAC payload, but it
* will _not_ contain the physical header or the MAC footer (MFR).
*
- * `payload_len` must be lower than the value retrieved when calling
+ * `payload_len` must be lower than or equal to the value retrieved when calling
* NETSTACK_RADIO.get_value(RADIO_CONST_MAX_PAYLOAD_LEN, ...)
*
* This function will return an error if the radio driver could not copy
