enable interrupt coalescing.

enable interrupt coalescing.

mvneta only supports coalescing tx completions by count, so this
uses the txmit value or half the ring as the threshold, whichever
is lower.

rx coalescing has a threshold on the number of packets, and a max
time it will wait before interrupting. however, it also has a
threshold for how many spare/empty descriptors there are on the
ring too. we use this latter threshold to have the chip interrupt
as soon as it uses the allocation that the rx ring moderation has
granted, and then first two to otherwise rate limit rx to about 4k
interrupts per second.

ok patrick@