RDMA/nes: Fix interrupt moderation low threshold

Interrupt moderation low threshold value was incorrectly triggering,
indicating that the threshold should be lowered.

The impact was the timer was likely to become 40usecs and get stuck
there.  The biggest side effect was too many interrupts and nonoptimal
performance.

Signed-off-by: John Lacombe <jlacombe@neteffect.com>
Signed-off-by: Glenn Streiff <gstreiff@neteffect.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>