igc: Increase default per-queue interrupt rate to 20000.

[grehan]

Summary

The default per-queue packet rate of 8000 will cause packet loss when
forwarding at 2.5G with a single stream, as is common when using e.g.
iperf3 to test a platform.

Bump this to 20000 (the "low latency" value in the Linux driver) which
avoids packet loss for this type of test.

Future work will use adaptive interrupt rate in a similar fashion
to the ixgbe driver.

Sponsored by: Rubicon Communications, LLC ("Netgate")

Test Plan

Forward iperf3 traffic through 2 igc ports at 2.5Gb/s

[erj]

It seems like a straightforward enough change to me.