rockminer: Limit even unsafe frequencies to 640 MHz, since above that overflows frequency bits and triggers fan control

driver-rockminer.c

@@ -22,7 +22,7 @@
 #define ROCKMINER_MIN_FREQ_MHZ  200
 #define ROCKMINER_DEF_FREQ_MHZ  270
 #define ROCKMINER_MAX_SAFE_FREQ_MHZ  290
-#define ROCKMINER_MAX_FREQ_MHZ  2560
+#define ROCKMINER_MAX_FREQ_MHZ  640
 #define ROCKMINER_POLL_US         0
 #define ROCKMINER_RETRY_US  5000000
 #define ROCKMINER_MIDTASK_TIMEOUT_US  500000