avr: Require 3500 H/s in hash test for starting difficulty of DUE

Boards like the BBC Microbit could only do ~2000 H/s at difficulty of
128, but it is assigned a difficulty of 512, which results in
immediate shares rejection.

As indicated by the naming, "ARM" as starting difficulty is available,
so use that instead.

Signed-off-by: Ricky Cheung <[email protected]>

Author: Ricky Cheung

AVR_Miner.py

@@ -1106,7 +1106,7 @@ def mine_avr(com, threadid, fastest_pool, thread_rigid):
             break
 
         start_diff = "AVR"
-        if hashrate_test > 1000:
+        if hashrate_test > 3500:
             start_diff = "DUE"
         elif hashrate_test > 550:
             start_diff = "ARM"