jtag_registers: Avalon bridge flushing tweaks

jtag_registers: Avalon bridge flushing tweaks

The code is now much more explicit. It flushes every
N writes. For now flush every time, but tinkering with
the bridge FIFO size and how often we flush clearly
points in the direction of the Avalon write FIFO full
being the culprit.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>