Diagnostics: Increase depth of timestamp FIFO
Currently, timestamps for incoming pulses are stored in a 128 deep ring
buffer.
For release 4 of the gateware, capable of supporting pulse repetition of
up to 2MHz, the fifo will fill up after 64us in the worst case.
Given the speed of the I2C bus, there is no possible way to access any
meaningful timestamp data in this setup.