adjust bm1370 mhz settings to 6.25 multiplier

[WantClue]

takes into account the 6.25 multiplier of the step algo for frequency PLL

[benjamin-wilson]

The default frequency is 596 on the antminers

[zcht]

While I'm looking at it, the 550 MHz step is still missing. The jump from 525 to 575 is extremely large at a gamma.