ftgmac100: Add more register inits in ftgmac100_init_hw()

Clear stale interrupts on entry, configure FIFO sizes, set FIFO
thresholds, configure interrupt mitigation.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>