hyperv/hn: Reduce TCP segment aggregation limit for multiple RX rings

This mainly used to improve ACK timeliness when multiple RX rings
are enabled.

This value gives the best performance in both Azure and Hyper-V
environment, w/ both 10Ge and 40Ge using non-{INVARIANTS,WITNESS}
kernel.

MFC after: 1 week
Sponsored by: Microsoft OSTC
Differential Revision: https://reviews.freebsd.org/D5691