e1000: fix em_mac_min and 82547 packet buffer

The boundary differentiating "lem" vs "em" class devices was wrong
after the iflib conversion of lem(4).

The Packet Buffer size for 82547 class chips was not set correctly
after the iflib conversion of lem(4).

These changes restore functionality on an 82547 for the submitter.

PR: 236119
Reported by: Jeff Gibbons <jgibbons@protogate.com>
Reviewed by: markj
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D29766