e1000: Increase FC pause/refresh time on PCH2 and newer

This corresponds to Linux f74dc880098b4a29f76d756b888fb31d81ad9a0c

That commit does not provide any public background detail, but it's been
in use for over 5 years and corresponds to previous chip bugs w.r.t.
automatic generation of PAUSE frames.

Reviewed by: kgalazka
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D54555