Fix the [no] downsampler border case for DSO 2250.

[uunicorn]

downsampler == 1 means divide by one => no downsampling, use the base clocks.

On 2250, frequency divider is a 16-bit up-counter. It outputs a pulse on overflow when enabled and reloads the value which was passed in the ESETTRIGGERORSAMPLERATE command.

If you enable downsampler with ESETTRIGGERORSAMPLERATE and:

• write 0xffff as a counter (0x10001 - 2), it will divide by 2
• write 0xfffe as a counter (0x10001 - 3), it will divide by 3 ....
• write 0x0001 as a counter (0x10001 - 0), it will divide by 65535
• write 0x0000 as a counter (0x10001 - 1), it will divide by 65536

Fixes #81

[davidgraeff]

Thanks :)