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

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")
Reviewed by: erj, kp
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D40904