Calculate checksum correctly in all cases

alpaca_isp/lpc.py

             # If we're writing sector 0, set the checksum
             if sector == 0:
                 iv = struct.unpack('<7I', secdat[0:28])
-                cs = struct.pack('<i', -sum(iv))
+                cs = struct.pack('<I', (-(sum(iv) % 2**32)) & 0xFFFFFFFF)
                 secdat = secdat[:28] + cs + secdat[32:]
 
             # Write data to RAM