bnxt_en: add support for software dynamic interrupt moderation

This implements the changes needed for the bnxt_en driver to add support
for dynamic interrupt moderation per ring.

This does add additional counters in the receive path, but testing shows
that any additional instructions are offset by throughput gain when the
default configuration is for low latency.

Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
Acked-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>